Multi-channel echo cancel method, multi-channel sound transfer method, stereo echo canceller, stereo sound transfer apparatus and transfer function calculation apparatus

ABSTRACT

Stereo sound signals are reproduced directly from loudspeakers (SP(L), SP(R)). By using a sum signal and a difference signal of the stereo sound signals as a reference signal, and according to a cross spectrum calculation of the reference signal with a microphone-collected sound signal, calculation is performed to obtain transfer functions of four sound transfer systems between the loudspeakers (SP(L), SP(R)) and microphones (MC(L), MC(R)). The transfer functions obtained are subjected to inverse Fourier transform to obtain impulse responses, which are set in filter means ( 40 - 1  to  40 - 4 ) to create echo cancel signals and perform echo canceling. This solves the problem of an indefinite coefficient in the echo cancel technique of a multi-channel sound signal.

RELATED APPLICATIONS

This application is a continuation of PCT application No.PCT/JP02/06968, filed Jul. 10, 2002, which is based upon, and claimspriority from, Japanese Patent Application No. 2001-211279, filed Jul.11, 2001 and Japanese Patent Application No. 2002-179722, filed Jun. 20,2002.

TECHNICAL FIELD

This invention relates to an echo cancellation technique formulti-channel audio signals and a transfer function calculationtechnique, and solves a problem of indefinite coefficient by a newtechnique.

BACKGROUND ART

In two-way stereo audio transmission that is used in teleconferencingsystems and so forth, the problem of indefinite coefficient of echocancellers has conventionally been pointed out and, for solving it,there have been proposed various techniques (see Journal of TheInstitute of Electronics, Information and Communication Engineers, vol.81, No. 3, pp. 266-274, March 1998). As one of the techniques forsolving the problem of indefinite coefficient, there is a method ofreducing the interchannel correlation. As concrete techniques therefor,there have conventionally been proposed the addition of random noise,the correlation removal by filters, the interchannel frequency shift,the use of an interleave comb filter, the nonlinear processing(Laid-Open Patent Publication No. H10-190848) and so forth.

According to the foregoing conventional techniques, since originalstereo signals are subjected to processing and then reproduced, therehas been a problem that deterioration more or less occurs in reproducedsignals. Further, when the processing is complicated, a delay occurs inthe reproduced signals, so that there has been a problem of difficultyin conversation in the teleconference and so forth. Further, when theprocessing is complicated and the processing capability of a processingcircuit is low, there have been those instances where it is difficult toupdate a coefficient of an echo canceller in real time while carryingout the echo cancel processing.

This invention provides an echo cancellation technique for multi-channelaudio signals and a transfer function calculation technique that havesolved the foregoing problems in the conventional techniques.

DISCLOSURE OF THE INVENTION

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, individual transfer functions of said plurality of audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimatedso as to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to corresponding individual signals to be reproduced bysaid respective loudspeakers or a plurality of composite signalsobtained by suitably combining said individual signals, and said echocancel signals are subtracted from corresponding individual collectedaudio signals of said one or plurality of microphones, or a plurality ofcomposite signals obtained by suitably combining said individualcollected audio signals, thereby performing echo cancellation, andwherein, using as reference signals (representing those signals that arereferred to for estimating the transfer functions or the compositetransfer functions) a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals (e.g. suitablycombining said multi-channel audio signals to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals and using a setof said plurality of low-correlation composite signals as referencesignals, or directly inputting a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that between said multi-channel audiosignals and using the set of said plurality of low-correlation compositesignals as reference signals, or the like), individual transferfunctions of the respective audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, usingas reference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, individual transfer functions of therespective audio transfer systems or a plurality of composite transferfunctions obtained by suitably combining such individual transferfunctions are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the individualtransfer functions of the respective audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, using as the reference signals theset of the plurality of low-correlation composite signals, may be, forexample, a calculation of respectively deriving the individual transferfunctions of the respective audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween the plurality of low-correlation composite signals and theindividual collected audio signals of the microphones, or the pluralityof composite signals obtained by suitably combining said individualcollected audio signals. Further, the calculation of respectivelyderiving the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the individual transfer functions of saidplurality of audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, by combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, individual transfer functions of said plurality of audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimatedso as to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to corresponding individual signals to be reproduced bysaid respective loudspeakers or a plurality of composite signalsobtained by suitably combining said individual signals, and said echocancel signals are subtracted from corresponding individual collectedaudio signals of said one or plurality of microphones, or a plurality ofcomposite signals obtained by suitably combining said individualcollected audio signals, thereby performing echo cancellation, wherein,using as reference signals a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that between said multi-channel audiosignals (e.g. suitably combining said multi-channel audio signals toproduce a plurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals and using a set of said plurality of low-correlation compositesignals as reference signals, or directly inputting a set of a pluralityof low-correlation composite signals which correspond to signalsobtained by suitably combining said multi-channel audio signals andwhich have a lower correlation with each other than that between saidmulti-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of individual transfer functions of the respectiveaudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions arerespectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, estimated errors ofindividual transfer functions of the respective audio transfer systemsor a plurality of composite transfer functions obtained by suitablycombining such individual transfer functions are respectively derived soas to successively update the corresponding filter characteristics tovalues that cancel such estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. Further, it is possible to update thefilter characteristics in real time. The calculation of respectivelyderiving the estimated errors of the individual transfer functions ofthe respective audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, using as the reference signals the set of theplurality of low-correlation composite signals, may be, for example, acalculation of respectively deriving the estimated errors of theindividual transfer functions of the respective audio transfer systemsor the plurality of composite transfer functions obtained by suitablycombining said individual transfer functions, based on a cross-spectrumcalculation between said plurality of low-correlation composite signalsand echo cancel error signals obtained by subtracting the echo cancelsignals from the corresponding individual collected audio signals ofsaid one or plurality of microphones, or the plurality of compositesignals obtained by suitably combining said individual collected audiosignals. Further, the calculation of respectively deriving the estimatederrors of the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the estimated errors of the individual transferfunctions of said plurality of audio transfer systems or the pluralityof composite transfer functions obtained by suitably combining saidindividual transfer functions, by combining said multi-channel audiosignals through addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe echo cancel error signals obtained by subtracting the echo cancelsignals from the corresponding individual collected audio signals ofsaid one or plurality of microphones, or the plurality of compositesignals obtained by suitably combining said individual collected audiosignals, and ensemble-averaging them in a predetermined time period percross spectrum. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

A multi-channel sound transfer method of this invention is such that,with respect to two spaces each forming said plurality of audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, individual transfer functions of said two or fouraudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions areestimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid set filter characteristics to corresponding individual signals tobe reproduced by said respective loudspeakers or a plurality ofcomposite signals obtained by suitably combining said individualsignals, and said echo cancel signals are subtracted from correspondingindividual collected audio signals of said one or two microphones, or aplurality of composite signals obtained by suitably combining saidindividual collected audio signals, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, individual transferfunctions of said two or four audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, sincethe sum signal and the difference signal of the stereo audio signalshave a low correlation therebetween, the transfer functions of the twoor four audio transfer systems or their composite transfer functions arerespectively derived using the sum signal and the difference signal asreference signals, so as to set the corresponding filtercharacteristics, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the individual transfer functions of said two or four audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, usingthe sum signal and the difference signal of said stereo audio signals asthe reference signals, may be, for example, a calculation ofrespectively deriving the individual transfer functions of said two orfour audio transfer systems or the plurality of composite transferfunctions obtained by suitably combining said individual transferfunctions, based on a cross-spectrum calculation between the sum signaland the difference signal, and the individual collected audio signals ofthe microphones, or the plurality of composite signals obtained bysuitably combining said individual collected audio signals. Further, thecalculation of respectively deriving the individual transfer functionsof said two or four audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, based on said cross-spectrum calculation, may be,for example, a calculation of respectively deriving the individualtransfer functions of said two or four audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, by deriving cross spectra betweenthe sum signal and the difference signal of said stereo audio signalsand the individual collected audio signals of the microphones or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, individual transfer functions of said two or fouraudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions areestimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid set filter characteristics to corresponding individual signals tobe reproduced by said respective loudspeakers or a plurality ofcomposite signals obtained by suitably combining said individualsignals, and said echo cancel signals are subtracted from correspondingindividual collected audio signals of said one or two microphones, or aplurality of composite signals obtained by suitably combining saidindividual collected audio signals, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, estimated errors ofindividual transfer functions of said two or four audio transfer systemsor a plurality of composite transfer functions obtained by suitablycombining said individual transfer functions are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, since the sumsignal and the difference signal of the stereo audio signals have a lowcorrelation therebetween, the estimated errors of the transfer functionsof the two or four audio transfer systems or their composite transferfunctions are respectively derived using the sum signal and thedifference signal as reference signals, so as to successively update thecorresponding filter characteristics to the values that cancel theestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the estimated errors of the individual transfer functions ofsaid two or four audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, using the sum signal and the difference signal ofsaid stereo audio signals as the reference signals, may be, for example,a calculation of respectively deriving the estimated errors of theindividual transfer functions of said two or four audio transfer systemsor the plurality of composite transfer functions obtained by suitablycombining said individual transfer functions, based on a cross-spectrumcalculation between the sum signal and the difference signal of saidstereo audio signals and respective echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the individualcollected audio signals of said one or two microphones, or the pluralityof composite signals obtained by suitably combining said individualcollected audio signals. The calculation of respectively deriving theestimated errors of the individual transfer functions of said two orfour audio transfer systems or the plurality of composite transferfunctions obtained by suitably combining said individual transferfunctions, based on the cross-spectrum calculation between the sumsignal and the difference signal of said stereo audio signals and saidecho cancel error signals, may be, for example, a calculation ofrespectively deriving the estimated errors of the individual transferfunctions of said two or four audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, by deriving cross spectra between the sumsignal and the difference signal of said stereo audio signals and saidecho cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum. Further, the correlationbetween the sum signal and the difference signal of said stereo audiosignals is detected and, when a value of said correlation is no lessthan a prescribed value, updating of said filter characteristics isstopped, thereby to prevent the echo cancel error signals fromunexpectedly increasing.

A stereo audio transmission method of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof the foregoing multi-channel echo cancel methods is carried out, sothat the stereo audio signals, which have been echo-canceled byperforming said method, are transmitted between said two spaces. Inaccordance therewith, the stereo audio transmission with reduced echocancellation can be performed between two spots, which, for example, canbe applied to the teleconferencing system or the like.

In this invention, the plurality of composite signals obtained bysuitably combining the individual signals to be reproduced by therespective loudspeakers, and the plurality of low-correlation compositesignals used as reference signals {in this specification,“low-correlation composite signals” is used as including the meaning ofuncorrelated signals (uncorrelated composite signals)} may be, forexample, common signals.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel sounds (e.g.multi-channel stereo sounds such as two-channel, four-channel, . . . )inputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, transfer functions of said plurality of audio transfersystems are estimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid filter characteristics to corresponding signals to be reproduced bysaid respective loudspeakers, and said echo cancel signals aresubtracted from corresponding collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),transfer functions of the respective audio transfer systems arerespectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the transfer functions of the respective audio transfer systemsare respectively derived, and corresponding filter characteristics areset, thereby to enable echo cancellation. In accordance therewith, sincethe multi-channel audio signals can be reproduced from the loudspeakerswith no or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the transfer functions of the respective audio transfer systemsusing as the reference signals the set of the plurality oflow-correlation composite signals, may be, for example, a calculation ofrespectively deriving the transfer functions of the respective audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the respectivemicrophone collected audio signals. Further, the calculation ofrespectively deriving the transfer functions of said plurality of audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe respective microphone collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum to derive a plurality of kinds of composite transfer functionsobtained by combining transfer functions of a plurality of suitablesystems among said plurality of audio transfer systems, thereby toderive transfer functions of said plurality of audio transfer systemsbased on said plurality of kinds of composite transfer functions.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, transfer functions of said plurality of audio transfersystems are estimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid filter characteristics to corresponding signals to be reproduced bysaid respective loudspeakers, and said echo cancel signals aresubtracted from corresponding collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of transfer functions of the respective audio transfersystems are respectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, the estimated errors ofthe transfer functions of the respective audio transfer systems arerespectively derived so as to successively update the correspondingfilter characteristics to the values that cancel said estimated errors,thereby to enable echo cancellation. In accordance therewith, since themulti-channel audio signals can be reproduced from the loudspeakers withno or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update thefilter characteristics in real time. The filter characteristics can beupdated, for example, per suitably determined prescribed time period(e.g. time period of performing said ensemble averaging). Thecalculation of respectively deriving the estimated errors of thetransfer functions of the respective audio transfer systems using theset of the plurality of low-correlation composite signals as thereference signals, may be, for example, a calculation of respectivelyderiving the estimated errors of the transfer functions of therespective audio transfer systems based on a cross-spectrum calculationbetween said plurality of low-correlation composite signals and echocancel error signals obtained by subtracting the corresponding echocancel signals from the collected audio signals of said one or pluralityof microphones. Further, the calculation of respectively deriving theestimated errors of the transfer functions of said plurality of audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe echo cancel error signals obtained by subtracting the correspondingecho cancel signals from the collected audio signals of said one orplurality of microphones, and ensemble-averaging them in a predeterminedtime period per cross spectrum to derive a plurality of kinds oftransfer function composite estimated errors obtained by combiningestimated errors of transfer functions of a plurality of suitablesystems among said plurality of audio transfer systems, thereby toderive estimated errors of the transfer functions of said plurality ofaudio transfer systems based on said plurality of kinds of transferfunction composite estimated errors. Further, the correlation betweensaid plurality of low-correlation composite signals is detected and,when a value of said correlation is no less than a prescribed value,updating of said filter characteristics is stopped, thereby to preventthe echo cancel error signals from unexpectedly increasing.

Further, the calculation of respectively deriving the transfer functionsof said plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of producing a pluralityof mutually orthogonal uncorrelated composite signals by applying aprincipal component analysis to said multi-channel audio signals,deriving cross spectra between said plurality of uncorrelated compositesignals and the respective microphone collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum, thereby to derive the transfer functions of said plurality ofaudio transfer systems based on the ensemble-averaged values.

Further, the calculation of respectively deriving the estimated errorsof the transfer functions of said plurality of audio transfer systemsbased on said cross-spectrum calculation, may be, for example, acalculation of producing a plurality of mutually orthogonal uncorrelatedcomposite signals by applying a principal component analysis to saidmulti-channel audio signals, deriving cross spectra between saidplurality of uncorrelated composite signals and the echo cancel errorsignals obtained by subtracting the corresponding echo cancel signalsfrom the collected audio signals of said one or plurality ofmicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum, thereby to derive the estimated errors of thetransfer functions of said plurality of audio transfer systems based onthe ensemble-averaged values. In this case, it may be arranged thatdouble talk in which sounds other than those reproduced by saidloudspeakers are inputted into said microphones is detected and, whenthe double talk is detected, an update period of said filtercharacteristics is made relatively longer, whereas, when the double talkis not detected, the update period of said filter characteristics ismade relatively shorter, so that it is possible to fully converge theestimated errors when the double talk exists, and further, quicken theconvergence of the estimated errors when there is no double talk.

A multi-channel sound transfer method of this invention is such that,with respect to two spaces each forming said plurality of audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, transfer functions of said two or four audiotransfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said filter characteristics to corresponding signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from corresponding collected audio signals ofsaid one or two microphones, thereby performing echo cancellation, andwherein, using a sum signal and a difference signal of said stereo audiosignals as reference signals, transfer functions of said two or fouraudio transfer systems are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, sincethe sum signal and the difference signal of the stereo audio signalshave a low correlation therebetween, the transfer functions of the twoor four audio transfer systems are respectively derived using the sumsignal and the difference signal as reference signals, so as to set thecorresponding filter characteristics, thereby to enable echocancellation. In accordance therewith, since the stereo signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the stereo signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the transferfunctions of said two or four audio transfer systems using the sumsignal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the transfer functions of said two or four audio transfersystems based on a cross-spectrum calculation between the sum signal andthe difference signal, and the respective microphone collected audiosignals. Further, the calculation of respectively deriving the transferfunctions of said two or four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between the sum signal and the difference signalof said stereo audio signals and the respective microphone collectedaudio signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum to derive a plurality of kinds of compositetransfer functions obtained by combining transfer functions of aplurality of suitable systems among said two or four audio transfersystems, thereby to derive transfer functions of said two or four audiotransfer systems based on said plurality of kinds of composite transferfunctions. Further, the cross-spectrum calculation in case of the foursystems may calculate, for example, respective cross spectra betweensaid sum signal and the first microphone collected audio signal, betweensaid sum signal and the second microphone collected audio signal,between said difference signal and the first microphone collected audiosignal, and between said difference signal and the second microphonecollected audio signal. Further, said composite transfer functions mayinclude, for example, the first composite transfer function that is thesum of a transfer function between the first loudspeaker and the firstmicrophone and a transfer function between the second loudspeaker andthe first microphone, the second composite transfer function that is adifference between the transfer function between the first loudspeakerand the first microphone and the transfer function between the secondloudspeaker and the first microphone, the third composite transferfunction that is the sum of a transfer function between the firstloudspeaker and the second microphone and a transfer function betweenthe second loudspeaker and the second microphone, and the fourthcomposite transfer function that is a difference between the transferfunction between the first loudspeaker and the second microphone and thetransfer function between the second loudspeaker and the secondmicrophone. The calculation of deriving the transfer functions of saidfour audio transfer systems may include, for example, a calculation ofderiving a transfer function of the first audio transfer system from thesum of the first composite transfer function and the second compositetransfer function, a calculation of deriving a transfer function of thesecond audio transfer system from a difference between the firstcomposite transfer function and the second composite transfer function,a calculation of deriving a transfer function of the third audiotransfer system from the sum of the third composite transfer functionand the fourth composite transfer function, and a calculation ofderiving a transfer function of the fourth audio transfer system from adifference between the third composite transfer function and the fourthcomposite transfer function.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, transfer functions of said two or four audiotransfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said filter characteristics to corresponding signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from corresponding collected audio signals ofsaid one or two microphones, thereby performing echo cancellation, andwherein, using a sum signal and a difference signal of said stereo audiosignals as reference signals, estimated errors of transfer functions ofsaid two or four audio transfer systems are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, since the sumsignal and the difference signal of the stereo audio signals have a lowcorrelation therebetween, the estimated errors of the transfer functionsof the two or four audio transfer systems are respectively derived usingthe sum signal and the difference signal as reference signals, so as tosuccessively update the corresponding filter characteristics to thevalues that cancel the estimated errors, thereby to enable echocancellation. In accordance therewith, since the stereo signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the stereo signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. It is also possible to update the echo cancel coefficients(filter characteristics) in real time. The filter characteristics can beupdated, for example, per suitably determined prescribed time period(e.g. time period of performing said ensemble averaging). Thecalculation of respectively deriving the estimated errors of thetransfer functions of said two or four audio transfer systems using thesum signal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the estimated errors of the transfer functions of said two orfour audio transfer systems based on a cross-spectrum calculationbetween the sum signal and the difference signal of said stereo audiosignals and respective echo cancel error signals obtained by subtractingthe corresponding echo cancel signals from the collected audio signalsof said one or two microphones. Further, the calculation of respectivelyderiving the estimated errors of the transfer functions of said two orfour audio transfer systems based on the cross-spectrum calculationbetween the sum signal and the difference signal of said stereo audiosignals and said echo cancel error signals, may be, for example, acalculation of deriving cross spectra between the sum signal and thedifference signal of said stereo audio signals and said echo cancelerror signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum to derive a plurality of kinds of transferfunction composite estimated errors obtained by combining estimatederrors of transfer functions of a plurality of suitable systems amongsaid two or four audio transfer systems, thereby to derive estimatederrors of the transfer functions of said two or four audio transfersystems based on said plurality of kinds of transfer function compositeestimated errors. Further, the cross-spectrum calculation in case of thefour systems may calculate, for example, respective cross spectrabetween said sum signal and the first echo cancel error signal, betweensaid sum signal and the second echo cancel error signal, between saiddifference signal and the first echo cancel error signal, and betweensaid difference signal and the second echo cancel error signal. Further,said transfer function composite estimated errors may include, forexample, the first transfer function composite estimated error that isthe sum of an estimated error of a transfer function between the firstloudspeaker and the first microphone and an estimated error of atransfer function between the second loudspeaker and the firstmicrophone, the second transfer function composite estimated error thatis a difference between the estimated error of the transfer functionbetween the first loudspeaker and the first microphone and the estimatederror of the transfer function between the second loudspeaker and thefirst microphone, the third transfer function composite estimated errorthat is the sum of an estimated error of a transfer function between thefirst loudspeaker and the second microphone and an estimated error of atransfer function between the second loudspeaker and the secondmicrophone, and the fourth transfer function composite estimated errorthat is a difference between the estimated error of the transferfunction between the first loudspeaker and the second microphone and theestimated error of the transfer function between the second loudspeakerand the second microphone. The calculation of deriving the estimatederrors of the transfer functions of said four audio transfer systems mayinclude, for example, a calculation of deriving an estimated error of atransfer function of the first audio transfer system from the sum of thefirst transfer function composite estimated error and the secondtransfer function composite estimated error, a calculation of derivingan estimated error of a transfer function of the second audio transfersystem from a difference between the first transfer function compositeestimated error and the second transfer function composite estimatederror, a calculation of deriving an estimated error of a transferfunction of the third audio transfer system from the sum of the thirdtransfer function composite estimated error and the fourth transferfunction composite estimated error, and a calculation of deriving anestimated error of a transfer function of the fourth audio transfersystem from a difference between the third transfer function compositeestimated error and the fourth transfer function composite estimatederror. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, transfer functions of said two or four audiotransfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said filter characteristics to corresponding signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from corresponding collected audio signals ofsaid one or two microphones, thereby performing echo cancellation, andwherein a principal component analysis is applied to said stereo audiosignals to produce two uncorrelated composite signals that areorthogonal to each other, and transfer functions of said two or fouraudio transfer systems are respectively derived using a set of said twouncorrelated composite signals as reference signals, thereby to setcorresponding filter characteristics. According to this invention, sincethe mutually orthogonal two signals produced by applying the principalcomponent analysis to the stereo audio signals are uncorrelated witheach other, the transfer functions of said two or four audio transfersystems are respectively derived using such two signals, thereby to setthe corresponding filter characteristics to enable echo cancellation. Inaccordance therewith, since the stereo signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the stereo signals, excellent reproduced tonequality can be achieved. Further, there is no or only a small delay inreproduced signals. Thus, when applying to the teleconferencing systemor the like, natural conversation can be conducted. The calculation ofrespectively deriving the transfer functions of said two or four audiotransfer systems using the set of said two uncorrelated compositesignals as the reference signals may be, for example, a calculation ofrespectively deriving transfer functions of said two or four audiotransfer systems based on a cross-spectrum calculation between said twouncorrelated composite signals and the respective microphone collectedaudio signals. Further, the calculation of respectively deriving thetransfer functions of said two or four audio transfer systems based onsaid cross-spectrum calculation may be, for example, a calculation ofderiving cross spectra between said two uncorrelated composite signalsand the respective microphone collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum derive a plurality of kinds of composite transfer functionsobtained by combining transfer functions of a plurality of suitablesystems among said two or four audio transfer systems, thereby to derivetransfer functions of said two or four audio transfer systems based onsaid plurality of kinds of composite transfer functions.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, transfer functions of said two or four audiotransfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said filter characteristics to corresponding signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from corresponding collected audio signals ofsaid one or two microphones, thereby performing echo cancellation, andwherein a principal component analysis is applied to said stereo audiosignals to produce two uncorrelated composite signals that areorthogonal to each other, and estimated errors of transfer functions ofsaid two or four audio transfer systems are respectively derived using aset of said two uncorrelated composite signals as reference signals,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, since themutually orthogonal two signals produced by applying the principalcomponent analysis to the stereo audio signals are uncorrelated witheach other, the estimated errors of the transfer functions of said twoor four audio transfer systems are respectively derived using such twosignals so as to successively update the corresponding filtercharacteristics to the values that cancel such estimated errors, therebyto enable echo cancellation. In accordance therewith, since the stereosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the stereo signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. It is also possible to update the echo cancel coefficients(filter characteristics) in real time. The filter characteristics can beupdated, for example, per suitably determined prescribed time period(e.g. time period of performing said ensemble averaging). Thecalculation of respectively deriving the estimated errors of thetransfer functions of said two or four audio transfer systems using theset of said two uncorrelated composite signals as the reference signalsmay be, for example, a calculation of respectively deriving estimatederrors of transfer functions of said two or four audio transfer systemsbased on a cross-spectrum calculation between said two uncorrelatedcomposite signals and respective echo cancel error signals obtained bysubtracting the corresponding echo cancel signals from the collectedaudio signals of said one or two microphones. Further, the calculationof respectively deriving the estimated errors of the transfer functionsof said two or four audio transfer systems based on the cross-spectrumcalculation between said two uncorrelated composite signals and saidecho cancel error signals, may be, for example, a calculation ofderiving cross spectra between said two uncorrelated composite signalsand said echo cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum to derive a plurality ofkinds of transfer function composite estimated errors obtained bycombining estimated errors of transfer functions of a plurality ofsuitable systems among said two or four audio transfer systems, therebyto derive estimated errors of the transfer functions of said two or fouraudio transfer systems based on said plurality of kinds of transferfunction composite estimated errors. In this case, it may be arrangedthat double talk in which sounds other than those reproduced by saidloudspeakers are inputted into said microphones is detected and, whenthe double talk is detected, an update period of said filtercharacteristics is made relatively longer, whereas, when the double talkis not detected, the update period of said filter characteristics ismade relatively shorter, so that it is possible to fully converge theestimated errors when the double talk exists, and further, quicken theconvergence of the estimated errors when there is no double talk.

A stereo audio transmission method of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof the foregoing multi-channel echo cancel methods is carried out, sothat the stereo audio signals, which have been echo-canceled byperforming said method, are transmitted between said two spaces. Inaccordance therewith, the stereo audio transmission with reduced echocancellation can be performed between two spots, which, for example, canbe applied to the teleconferencing system or the like.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, which are provided corresponding to thefirst and second microphones, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, which are provided corresponding to the first and secondmicrophones, so as to produce third and fourth echo cancel signals, echocancellation is performed by subtracting, using first subtracting means,said first and third echo cancel signals from a collected audio signalof the first microphone, and echo cancellation is performed bysubtracting, using second subtracting means, said second and fourth echocancel signals from a collected audio signal of the second microphone,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving filter characteristics corresponding totransfer functions of said four audio transfer systems based on across-spectrum calculation between a sum signal and a difference signalof stereo audio signals to be reproduced by said respective loudspeakersand the collected audio signals of said respective microphones, therebyto set said derived filter characteristics to corresponding ones of saidfirst to fourth filter means, respectively. It may be arranged, forexample, that the stereo echo canceller of this invention comprisesinput means for inputting said stereo audio signals; sum/differencesignal producing means for producing a sum signal and a differencesignal of the stereo audio signals inputted from said input means; and amain signal transmission system for transmitting the stereo audiosignals inputted from said input means to said respective loudspeakersnot through said sum/difference signal producing means, wherein saidtransfer function calculating means derives the filter characteristicscorresponding to the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between the sum signaland the difference signal produced by said sum/difference signalproducing means and the respective microphone collected audio signals,and sets the derived filter characteristics to corresponding ones ofsaid first to fourth filter means, respectively. Alternatively, it maybe arranged that the stereo echo canceller comprises input means forinputting said stereo audio signals; sum/difference signal producingmeans for producing a sum signal and a difference signal of the stereoaudio signals inputted from said input means; and stereo audio signaldemodulating means for calculating the sum of and a difference betweenthe sum signal and the difference signal produced by said sum/differencesignal producing means so as to recover the original stereo audiosignals, wherein the stereo audio signals recovered by said stereo audiosignal demodulating means is transmitted to said respectiveloudspeakers, and said transfer function calculating means derives thefilter characteristics corresponding to the transfer functions of saidfour audio transfer systems based on the cross-spectrum calculationbetween the sum signal and the difference signal produced by saidsum/difference signal producing means and the respective microphonecollected audio signals, and sets the derived filter characteristics tocorresponding ones of said first to fourth filter means, respectively.Alternatively, it may also be arranged that the stereo echo cancellercomprises input means for inputting a sum signal and a difference signalof said stereo audio signals; and stereo audio signal demodulating meansfor calculating the sum of and a difference between said inputted sumsignal and difference signal so as to recover the original stereo audiosignals, wherein the stereo audio signals recovered by said stereo audiosignal demodulating means is transmitted to said respectiveloudspeakers, and said transfer function calculating means derives thefilter characteristics corresponding to the transfer functions of saidfour audio transfer systems based on the cross-spectrum calculationbetween said inputted sum signal and difference signal and therespective microphone collected audio signals, and sets the derivedfilter characteristics to corresponding ones of said first to fourthfilter means, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, which are provided corresponding to thefirst and second microphones, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, which are provided corresponding to the first and secondmicrophones, so as to produce third and fourth echo cancel signals, echocancellation is performed by subtracting, using first subtracting means,said first and third echo cancel signals from a collected audio signalof the first microphone, and echo cancellation is performed bysubtracting, using second subtracting means, said second and fourth echocancel signals from a collected audio signal of the second microphone,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving estimated errors of transfer functionsof said four audio transfer systems based on a cross-spectrumcalculation between a sum signal and a difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and respectiveecho cancel error signals obtained by subtracting the corresponding echocancel signals from the collected audio signals of said two microphones,thereby to update filter characteristics of said first to fourth filtermeans to values that cancel said estimated errors, respectively. It maybe arranged, for example, that the stereo echo canceller of thisinvention comprises input means for inputting said stereo audio signals;sum/difference signal producing means for producing a sum signal and adifference signal of the stereo audio signals inputted from said inputmeans; and a main signal transmission system for transmitting the stereoaudio signals inputted from said input means to said respectiveloudspeakers not through said sum/difference signal producing means,wherein said transfer function calculating means derives the estimatederrors of the transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the sum signal and thedifference signal produced by said sum/difference signal producing meansand the respective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively. Alternatively, it may bearranged that the stereo echo canceller comprises input means forinputting said stereo audio signals; sum/difference signal producingmeans for producing a sum signal and a difference signal of the stereoaudio signals inputted from said input means; and stereo audio signaldemodulating means for calculating the sum of and a difference betweenthe sum signal and the difference signal produced by said sum/differencesignal producing means so as to recover the original stereo audiosignals, wherein the stereo audio signals recovered by said stereo audiosignal demodulating means is transmitted to said respectiveloudspeakers, and said transfer function calculating means derives theestimated errors of the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between the sum signaland the difference signal produced by said sum/difference signalproducing means and the respective echo cancel error signals, andupdates the filter characteristics of said first to fourth filter meansto the values that cancel said estimated errors, respectively.Alternatively, it may also be arranged that the stereo echo cancellercomprises input means for inputting a sum signal and a difference signalof said stereo audio signals; and stereo audio signal demodulating meansfor calculating the sum of and a difference between said inputted sumsignal and difference signal so as to recover the original stereo audiosignals, wherein the stereo audio signals recovered by said stereo audiosignal demodulating means is transmitted to said respectiveloudspeakers, and said transfer function calculating means derives theestimated errors of the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between said inputtedsum signal and difference signal produced by said sum/difference signalproducing means and the respective echo cancel error signals, andupdates the filter characteristics of said first to fourth filter meansto the values that cancel said estimated errors, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, which are provided corresponding to thefirst and second microphones, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, which are provided corresponding to the first and secondmicrophones, so as to produce third and fourth echo cancel signals, echocancellation is performed by subtracting, using first subtracting means,said first and third echo cancel signals from a collected audio signalof the first microphone, and echo cancellation is performed bysubtracting, using second subtracting means, said second and fourth echocancel signals from a collected audio signal of the second microphone,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving filter characteristics corresponding totransfer functions of said four audio transfer systems based on across-spectrum calculation between mutually orthogonal two uncorrelatedcomposite signals produced by applying a principal component analysis tostereo audio signals to be reproduced by said respective loudspeakersand the respective microphone collected audio signals, thereby to setsaid derived filter characteristics to corresponding ones of said firstto fourth filter means, respectively. It may be arranged, for example,that the stereo echo canceller of this invention comprises input meansfor inputting said stereo audio signals; orthogonalizing means forapplying a principal component analysis to the stereo audio signalsinputted from said input means to produce mutually orthogonal twouncorrelated composite signals; and a main signal transmission systemfor transmitting the stereo audio signals inputted from said input meansto said respective loudspeakers not through said orthogonalizing means,wherein said transfer function calculating means derives the filtercharacteristics corresponding to the transfer functions of said fouraudio transfer systems based on the cross-spectrum calculation betweenthe two uncorrelated composite signals produced by said orthogonalizingmeans and the respective microphone collected audio signals, and setsthe derived filter characteristics to corresponding ones of said firstto fourth filter means, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, which are provided corresponding to thefirst and second microphones, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, which are provided corresponding to the first and secondmicrophones, so as to produce third and fourth echo cancel signals, echocancellation is performed by subtracting, using first subtracting means,said first and third echo cancel signals from a collected audio signalof the first microphone, and echo cancellation is performed bysubtracting, using second subtracting means, said second and fourth echocancel signals from a collected audio signal of the second microphone,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving estimated errors of transfer functionsof said four audio transfer systems based on a cross-spectrumcalculation between mutually orthogonal two uncorrelated compositesignals produced by applying a principal component analysis to stereoaudio signals to be reproduced by said respective loudspeakers andrespective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the collected audio signals ofsaid two microphones, thereby to update filter characteristics of saidfirst to fourth filter means to values that cancel said estimatederrors, respectively. It may be arranged, for example, that the stereoecho canceller of this invention comprises input means for inputtingsaid stereo audio signals; orthogonalizing means for applying aprincipal component analysis to the stereo audio signals inputted fromsaid input means to produce mutually orthogonal two uncorrelatedcomposite signals; and a main signal transmission system fortransmitting the stereo audio signals inputted from said input means tosaid respective loudspeakers not through said orthogonalizing means,wherein said transfer function calculating means derives the estimatederrors of the transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the two uncorrelatedcomposite signals produced by said orthogonalizing means and therespective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively. In this case, it may bearranged that double talk detecting means is provided for detectingdouble talk in which sounds other than those reproduced by saidloudspeakers are inputted into said microphones and, when the doubletalk is detected, said transfer function calculating means makesrelatively longer an update period of said filter characteristics,whereas, when the double talk is not detected, it makes relativelyshorter the update period of said filter characteristics, so that it ispossible to fully converge the estimated errors when the double talkexists, and further, quicken the convergence of the estimated errorswhen there is no double talk.

The stereo echo canceller of this invention may be further provided withcorrelation detecting means for detecting the correlation between thesum signal and the difference signal of said stereo audio signals and,when a value of said correlation is no less than a prescribed value,stopping updating of said filter characteristics, thereby to prevent theecho cancel error signals from unexpectedly increasing.

A stereo sound transfer apparatus of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof said stereo echo cancellers is arranged in each space, so that thestereo audio signals, which have been echo-canceled by said stereo echocancellers, are transmitted between said two spaces.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),composite transfer functions of said plurality of audio transfer systemsare respectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the composite transfer functions of said plurality of audiotransfer systems are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said plurality of audio transfer systems using asthe reference signals the set of the plurality of low-correlationcomposite signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said plurality of audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the individualcollected audio signals of the respective microphones. Further, thecalculation of respectively deriving the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the individual collected audiosignals of the respective microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive compositetransfer functions of said plurality of audio transfer systems.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to updatecorresponding filter characteristics to values that cancel saidestimated errors. According to this invention, using as referencesignals a set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems are respectively derived so asto successively update the corresponding filter characteristics to thevalues that cancel said estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. It is also possible to update the filtercharacteristics in real time. The calculation of respectively derivingthe estimated errors of the composite transfer functions of saidplurality of audio transfer systems using the set of the plurality oflow-correlation composite signals as the reference signals, may be, forexample, a calculation of respectively deriving the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the individualcollected audio signals of said one or plurality of microphones.Further, the calculation of respectively deriving the estimated errorsof the composite transfer functions of said plurality of audio transfersystems based on said cross-spectrum calculation, may be, for example, acalculation of combining said multi-channel audio signals throughaddition or subtraction to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals, deriving cross spectra betweensaid plurality of low-correlation composite signals and the echo cancelerror signals obtained by subtracting the corresponding echo cancelsignals from the individual collected audio signals of said one orplurality of microphones, and ensemble-averaging them in a predeterminedtime period per cross spectrum to derive estimated errors of thecomposite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

A multi-channel sound transfer method of this invention is such that,with respect to two spaces each forming said plurality of audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to composite signalsof individual signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from individual collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, composite transferfunctions of said two or four audio transfer systems are respectivelyderived, thereby to set corresponding filter characteristics. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, the compositetransfer functions of the two or four audio transfer systems arerespectively derived using the sum signal and the difference signal asreference signals, so as to set the corresponding filtercharacteristics, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems using the sum signal and the difference signal of saidstereo audio signals as the reference signals, may be, for example, acalculation of respectively deriving the composite transfer functions ofsaid two or four audio transfer systems based on a cross-spectrumcalculation between the sum signal and the difference signal, and theindividual collected audio signals of the respective microphones.Further, the calculation of respectively deriving the composite transferfunctions of said two or four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between the sum signal and the difference signalof said stereo audio signals and the individual collected audio signalsof the respective microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive compositetransfer functions of said two or four audio transfer systems.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to composite signalsof individual signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from individual collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, estimated errors ofcomposite transfer functions of said two or four audio transfer systemsare respectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, the estimatederrors of the composite transfer functions of the two or four audiotransfer systems are respectively derived using the sum signal and thedifference signal as reference signals, so as to successively update thecorresponding filter characteristics to the values that cancel theestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid two or four audio transfer systems using the sum signal and thedifference signal of said stereo audio signals as the reference signals,may be, for example, a calculation of respectively deriving theestimated errors of the composite transfer functions of said two or fouraudio transfer systems based on a cross-spectrum calculation between thesum signal and the difference signal of said stereo audio signals andrespective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the individual collected audiosignals of said one or two microphones. Further, the calculation ofrespectively deriving the estimated errors of the composite transferfunctions of said two or four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and said echo cancel error signals,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andsaid echo cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said two or four audio transfersystems. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

A stereo audio transmission method of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof the foregoing multi-channel echo cancel methods is carried out, sothat the stereo audio signals, which have been echo-canceled byperforming said method, are transmitted between said two spaces. Inaccordance therewith, the stereo audio transmission with reduced echocancellation can be performed between two spots, which, for example, canbe applied to the teleconferencing system or the like.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, a sum signal of stereo audio signals tobe reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a collected audio signal of the first microphone, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a collected audiosignal of the second microphone, said stereo echo canceller comprising:transfer function calculating means for respectively deriving filtercharacteristics corresponding to composite transfer functions of saidfour audio transfer systems based on a cross-spectrum calculationbetween the sum signal and the difference signal of the stereo audiosignals to be reproduced by said respective loudspeakers and therespective microphone collected audio signals, thereby to set saidderived filter characteristics to corresponding ones of said first tofourth filter means, respectively. It may be arranged, for example, thatthe stereo echo canceller of this invention comprises input means forinputting said stereo audio signals; sum/difference signal producingmeans for producing a sum signal and a difference signal of the stereoaudio signals inputted from said input means; and a main signaltransmission system for transmitting the stereo audio signals inputtedfrom said input means to said respective loudspeakers not through saidsum/difference signal producing means, wherein said transfer functioncalculating means derives the filter characteristics corresponding tothe composite transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the sum signal and thedifference signal produced by said sum/difference signal producing meansand the respective microphone collected audio signals, and sets thederived filter characteristics to corresponding ones of said first tofourth filter means, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, a sum signal of stereo audio signals tobe reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a collected audio signal of the first microphone, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a collected audiosignal of the second microphone, said stereo echo canceller comprising:transfer function calculating means for respectively deriving estimatederrors of composite transfer functions of said four audio transfersystems based on a cross-spectrum calculation between the sum signal andthe difference signal of the stereo audio signals to be reproduced bysaid respective loudspeakers and respective echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecollected audio signals of said two microphones, thereby to updatefilter characteristics of said first to fourth filter means to valuesthat cancel said estimated errors, respectively. It may be arranged, forexample, that the stereo echo canceller of this invention comprisesinput means for inputting said stereo audio signals; sum/differencesignal producing means for producing a sum signal and a differencesignal of the stereo audio signals inputted from said input means; and amain signal transmission system for transmitting the stereo audiosignals inputted from said input means to said respective loudspeakersnot through said sum/difference signal producing means, wherein saidtransfer function calculating means derives the estimated errors of thecomposite transfer functions of said four audio transfer systems basedon the cross-spectrum calculation between the sum signal and thedifference signal produced by said sum/difference signal producing meansand the respective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively.

The stereo echo canceller of this invention may be further provided withcorrelation detecting means for detecting the correlation between thesum signal and the difference signal of said stereo audio signals and,when a value of said correlation is no less than a prescribed value,stopping updating of said filter characteristics, thereby to prevent theecho cancel error signals from unexpectedly increasing.

A stereo sound transfer apparatus of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof said stereo echo cancellers is arranged in each space, so that thestereo audio signals, which have been echo-canceled by said stereo echocancellers, are transmitted between said two spaces.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor plurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),composite transfer functions of said plurality of audio transfer systemsare respectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the composite transfer functions of said plurality of audiotransfer systems are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said plurality of audio transfer systems using asthe reference signals the set of the plurality of low-correlationcomposite signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said plurality of audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the composite signalsof the individual collected audio signals of the respective microphones.Further, the calculation of respectively deriving the composite transferfunctions of said plurality of audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofcombining said multi-channel audio signals through addition orsubtraction to produce a plurality of low-correlation composite signalshaving a lower correlation with each other than that between saidmulti-channel audio signals, deriving cross spectra between saidplurality of low-correlation composite signals and the composite signalsof the individual collected audio signals of the respective microphones,and ensemble-averaging them in a predetermined time period per crossspectrum to derive composite transfer functions of said plurality ofaudio transfer systems.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor plurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to updatecorresponding filter characteristics to values that cancel saidestimated errors. According to this invention, using as referencesignals a set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems are respectively derived so asto successively update the corresponding filter characteristics to thevalues that cancel said estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. It is also possible to update the filtercharacteristics in real time. The calculation of respectively derivingthe estimated errors of the composite transfer functions of saidplurality of audio transfer systems using the set of the plurality oflow-correlation composite signals as the reference signals, may be, forexample, a calculation of respectively deriving the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the compositesignals of the individual collected audio signals of said one orplurality of microphones. Further, the calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecomposite signals of the individual collected audio signals of said oneor plurality of microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

A multi-channel sound transfer method of this invention is such that,with respect to two spaces each forming said plurality of audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to individual signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from composite signals of individual collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, composite transferfunctions of said two or four audio transfer systems are respectivelyderived, thereby to set corresponding filter characteristics. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, estimatederrors of the composite transfer functions of the two or four audiotransfer systems are respectively derived using the sum signal and thedifference signal as reference signals, so as to successively update thecorresponding filter characteristics to values that cancel saidestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems using the sum signal and the difference signal of saidstereo audio signals as the reference signals, may be, for example, acalculation of respectively deriving the composite transfer functions ofsaid two or four audio transfer systems based on a cross-spectrumcalculation between the sum signal and the difference signal, and thecomposite signals of the individual collected audio signals of therespective microphones. Further, the calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of deriving cross spectra between the sum signaland the difference signal of said stereo audio signals and the compositesignals of the individual collected audio signals of the respectivemicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum to derive composite transfer functions of said two orfour audio transfer systems.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to individual signalsto be reproduced by said respective loudspeakers, and said echo cancelsignals are subtracted from composite signals of individual collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, estimated errors ofcomposite transfer functions of said two or four audio transfer systemsare respectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems are respectively derived, so as to successively update thecorresponding filter characteristics to the values that cancel theestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the multi-channel audio signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the multi-channel audio signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. It is also possible to update the filter characteristics inreal time. The calculation of respectively deriving the estimated errorsof the composite transfer functions of said two or four audio transfersystems using the sum signal and the difference signal of said stereoaudio signals as the reference signals, may be, for example, acalculation of respectively deriving the estimated errors of thecomposite transfer functions of said two or four audio transfer systemsbased on a cross-spectrum calculation between the sum signal and thedifference signal of said stereo audio signals and respective echocancel error signals obtained by subtracting the corresponding echocancel signals from the composite signals of the individual collectedaudio signals of said one or two microphones. Further, the calculationof respectively deriving the estimated errors of the composite transferfunctions of said two or four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and said echo cancel error signals,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andsaid echo cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said two or four audio transfersystems. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

A stereo audio transmission method of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof the foregoing multi-channel echo cancel methods is carried out, sothat the stereo audio signals, which have been echo-canceled byperforming said method, are transmitted between said two spaces. Inaccordance therewith, the stereo audio transmission with reduced echocancellation can be performed between two spots, which, for example, canbe applied to the teleconferencing system or the like.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, so as to produce third and fourth echo cancel signals,echo cancellation is performed by subtracting, using first subtractingmeans, said first and third echo cancel signals from a sum signal ofcollected audio signals of the respective microphones, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a differencesignal of the collected audio signals of the respective microphones,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving filter characteristics corresponding tocomposite transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between a sum signal and a differencesignal of stereo audio signals to be reproduced by said respectiveloudspeakers and the sum signal and the difference signal of therespective microphone collected audio signals, thereby to set saidderived filter characteristics to corresponding ones of said first tofourth filter means, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, an audio signal supplied to the firstloudspeaker is subjected to convolution calculations by first and secondfilter means, respectively, so as to produce first and second echocancel signals, an audio signal supplied to the second loudspeaker issubjected to convolution calculations by third and fourth filter means,respectively, so as to produce third and fourth echo cancel signals,echo cancellation is performed by subtracting, using first subtractingmeans, said first and third echo cancel signals from a sum signal ofcollected audio signals of the respective microphones, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a differencesignal of the collected audio signals of the respective microphones,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving estimated errors of composite transferfunctions of said four audio transfer systems based on a cross-spectrumcalculation between a sum signal and a difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and respectiveecho cancel error signals obtained by subtracting the corresponding echocancel signals from the sum signal and the difference signal of therespective microphone collected audio signals, thereby to update filtercharacteristics of said first to fourth filter means to values thatcancel said estimated errors, respectively.

The stereo echo canceller of this invention may be further provided withcorrelation detecting means for detecting the correlation between thesum signal and the difference signal of said stereo audio signals and,when a value of said correlation is no less than a prescribed value,stopping updating of said filter characteristics, thereby to prevent theecho cancel error signals from unexpectedly increasing.

A stereo sound transfer apparatus of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof said stereo echo cancellers is arranged in each space, so that thestereo audio signals, which have been echo-canceled by said stereo echocancellers, are transmitted between said two spaces.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or plurality of microphones, thereby performing echocancellation, and wherein, using as reference signals a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, usingas reference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, the composite transfer functions of saidplurality of audio transfer systems are respectively derived, andcorresponding filter characteristics are set, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. The calculation of respectively derivingthe composite transfer functions of said plurality of audio transfersystems using as the reference signals the set of the plurality oflow-correlation composite signals, may be, for example, a calculation ofrespectively deriving the composite transfer functions of said pluralityof audio transfer systems based on a cross-spectrum calculation betweenthe plurality of low-correlation composite signals and the compositesignals of the individual collected audio signals of the respectivemicrophones. Further, the calculation of respectively deriving thecomposite transfer functions of said plurality of audio transfer systemsbased on said cross-spectrum calculation, may be, for example, acalculation of combining said multi-channel audio signals throughaddition or subtraction to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals, deriving cross spectra betweensaid plurality of low-correlation composite signals and the compositesignals of the individual collected audio signals of the respectivemicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum to derive composite transfer functions of saidplurality of audio transfer systems.

A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or plurality of microphones, thereby performing echocancellation, and wherein, using as reference signals a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), estimated errors of composite transfer functionsof said plurality of audio transfer systems are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, using asreference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, the estimated errors of the compositetransfer functions of said plurality of audio transfer systems arerespectively derived so as to successively update the correspondingfilter characteristics to the values that cancel said estimated errors,thereby to enable echo cancellation. In accordance therewith, since themulti-channel audio signals can be reproduced from the loudspeakers withno or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update thefilter characteristics in real time. The calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems using the set of the pluralityof low-correlation composite signals as the reference signals, may be,for example, a calculation of respectively deriving the estimated errorsof the composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the compositesignals of the individual collected audio signals of said one orplurality of microphones. Further, the calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecomposite signals of the individual collected audio signals of said oneor plurality of microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

A multi-channel sound transfer method of this invention is such that,with respect to two spaces each forming said plurality of audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to composite signalsof individual signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from composite signals ofindividual collected audio signals of said one or two microphones,thereby performing echo cancellation, and wherein, using a sum signaland a difference signal of said stereo audio signals as referencesignals, composite transfer functions of said two or four audio transfersystems are respectively derived, thereby to set corresponding filtercharacteristics. According to this invention, since the sum signal andthe difference signal of the stereo audio signals have a low correlationtherebetween, estimated errors of the composite transfer functions ofthe two or four audio transfer systems are respectively derived usingthe sum signal and the difference signal as reference signals, so as tosuccessively update the corresponding filter characteristics to valuesthat cancel said estimated errors, thereby to enable echo cancellation.In accordance therewith, since the stereo signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the stereo signals, excellent reproduced tonequality can be achieved. Further, there is no or only a small delay inreproduced signals. Thus, when applying to the teleconferencing systemor the like, natural conversation can be conducted. The calculation ofrespectively deriving the composite transfer functions of said two orfour audio transfer systems using the sum signal and the differencesignal of said stereo audio signals as the reference signals, may be,for example, a calculation of respectively deriving the compositetransfer functions of said two or four audio transfer systems based on across-spectrum calculation between the sum signal and the differencesignal, and the composite signals of the individual collected audiosignals of the respective microphones. Further, the calculation ofrespectively deriving the composite transfer functions of said two orfour audio transfer systems based on said cross-spectrum calculation,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andthe composite signals of the individual collected audio signals of therespective microphones, and ensemble-averaging them in a predeterminedtime period per cross spectrum to derive composite transfer functions ofsaid two or four audio transfer systems.

A stereo echo cancel method of this invention is a method wherein, withrespect to a space provided therein with two loudspeakers and one or twomicrophones and forming two or four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said microphones, composite transfer functions of said two or fouraudio transfer systems are estimated so as to set corresponding filtercharacteristics, respectively, echo cancel signals are respectivelyproduced by giving said set filter characteristics to composite signalsof individual signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from composite signals ofindividual collected audio signals of said one or two microphones,thereby performing echo cancellation, and wherein, using a sum signaland a difference signal of said stereo audio signals as referencesignals, estimated errors of composite transfer functions of said two orfour audio transfer systems are respectively derived, thereby to updatecorresponding filter characteristics to values that cancel saidestimated errors. According to this invention, using as referencesignals a set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems are respectively derived, so asto successively update the corresponding filter characteristics to thevalues that cancel the estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. It is also possible to update the filtercharacteristics in real time. The calculation of respectively derivingthe estimated errors of the composite transfer functions of said two orfour audio transfer systems using the sum signal and the differencesignal of said stereo audio signals as the reference signals, may be,for example, a calculation of respectively deriving the estimated errorsof the composite transfer functions of said two or four audio transfersystems based on a cross-spectrum calculation between the sum signal andthe difference signal of said stereo audio signals and respective echocancel error signals obtained by subtracting the corresponding echocancel signals from the composite signals of the individual collectedaudio signals of said one or two microphones. Further, the calculationof respectively deriving the estimated errors of the composite transferfunctions of said two or four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and said echo cancel error signals,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andsaid echo cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said two or four audio transfersystems. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

A stereo audio transmission method of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof the foregoing multi-channel echo cancel methods is carried out, sothat the stereo audio signals, which have been echo-canceled byperforming said method, are transmitted between said two spaces. Inaccordance therewith, the stereo audio transmission with reduced echocancellation can be performed between two spots, which, for example, canbe applied to the teleconferencing system or the like.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, a sum signal of stereo audio signals tobe reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones, and echo cancellation is performed by subtracting, usingsecond subtracting means, said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving filtercharacteristics corresponding to composite transfer functions of saidfour audio transfer systems based on a cross-spectrum calculationbetween the sum signal and the difference signal of the stereo audiosignals to be reproduced by said respective loudspeakers and the sumsignal and the difference signal of the respective microphone collectedaudio signals, thereby to set said derived filter characteristics tocorresponding ones of said first to fourth filter means, respectively.

A stereo echo canceller of this invention is a stereo echo cancellerwherein, with respect to a space provided therein with two loudspeakersand two microphones and forming four audio transfer systems in whichstereo sounds reproduced by said respective loudspeakers are collectedby said respective microphones, a sum signal of stereo audio signals tobe reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones, and echo cancellation is performed by subtracting, usingsecond subtracting means, said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving estimated errors ofcomposite transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between the sum signal and thedifference signal of the stereo audio signals to be reproduced by saidrespective loudspeakers and respective echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thesum signal and the difference signal of the respective microphonecollected audio signals, thereby to update filter characteristics ofsaid first to fourth filter means to values that cancel said estimatederrors, respectively.

The stereo echo canceller of this invention may be further provided withcorrelation detecting means for detecting the correlation between thesum signal and the difference signal of said stereo audio signals and,when a value of said correlation is no less than a prescribed value,stopping updating of said filter characteristics, thereby to prevent theecho cancel error signals from unexpectedly increasing.

A stereo sound transfer apparatus of this invention is such that, withrespect to two spaces each forming said four audio transfer systems, anyof said stereo echo cancellers is arranged in each space, so that thestereo audio signals, which have been echo-canceled by said stereo echocancellers, are transmitted between said two spaces.

A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with a plurality of loudspeakers and one or a pluralityof microphones and forming a plurality of audio transfer systems inwhich multi-channel sounds inputted from an outside and reproduced bysaid respective loudspeakers and having a correlation with each otherare collected by said microphones, estimates individual transferfunctions of said plurality of audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, wherein, using as reference signals a setof a plurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), individual transfer functions of the respectiveaudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions areestimated. The calculation of respectively deriving the individualtransfer functions of the respective audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, using as the reference signals theset of the plurality of low-correlation composite signals, may be, forexample, a calculation of respectively deriving the individual transferfunctions of the respective audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween the plurality of low-correlation composite signals and theindividual collected audio signals of the microphones, or the pluralityof composite signals obtained by suitably combining said individualcollected audio signals. Further, the calculation of respectivelyderiving the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the individual transfer functions of saidplurality of audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, by combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum. The calculation ofrespectively deriving the individual transfer functions of saidplurality of audio transfer systems based on said cross-spectrumcalculation may also be a calculation of respectively deriving theindividual transfer functions of said plurality of audio transfersystems by producing a plurality of mutually orthogonal uncorrelatedcomposite signals by applying a principal component analysis to saidmulti-channel audio signals, deriving cross spectra between saidplurality of uncorrelated composite signals and the individual collectedaudio signals of the microphones, and ensemble-averaging them in apredetermined time period per cross spectrum.

A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with two loudspeakers and two microphones and formingfour audio transfer systems in which stereo sounds reproduced by saidrespective loudspeakers are collected by said respective microphones,estimates individual transfer functions of said four audio transfersystems or a plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, wherein, using asum signal and a difference signal of said stereo audio signals asreference signals, individual transfer functions of said four audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimated.The calculation of respectively deriving the individual transferfunctions of said four audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, using the sum signal and the differencesignal of said stereo audio signals as the reference signals, may be,for example, a calculation of respectively deriving the individualtransfer functions of said four audio transfer systems or the pluralityof composite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween said sum signal and said difference signal, and individualcollected audio signals of the microphones, or a plurality of compositesignals obtained by suitably combining said individual collected audiosignals. Further, the calculation of respectively deriving theindividual transfer functions of said four audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, based on said cross-spectrumcalculation, may be a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum, thereby to respectivelyderive the individual transfer functions of said four audio transfersystems or the plurality of composite transfer functions obtained bysuitably combining said individual transfer functions.

A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with two loudspeakers and two microphones and formingfour audio transfer systems in which stereo sounds reproduced by saidrespective loudspeakers are collected by said respective microphones,estimates individual transfer functions of said four audio transfersystems, wherein mutually orthogonal two uncorrelated composite signalsare produced by applying a principal component analysis to said stereoaudio signals, and individual transfer functions of said four audiotransfer systems are estimated using a set of said two uncorrelatedcomposite signals as reference signals. The calculation of respectivelyderiving the individual transfer functions of said four audio transfersystems using said two uncorrelated composite signals as the referencesignals, may be, for example, a calculation of respectively deriving theindividual transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between said two uncorrelated compositesignals and the individual collected audio signals of the microphones.Further, the calculation of respectively deriving the individualtransfer functions of said four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between said two uncorrelated composite signalsand the individual collected audio signals of the microphones, andensemble-averaging them in a predetermined time period per crossspectrum, thereby to respectively derive the individual transferfunctions of said four audio transfer systems. In this case, by makingrelatively longer a time period of performing said ensemble averagingwhen double talk is detected where sounds other than those reproduced bysaid loudspeakers are inputted into said microphones, while making itrelatively shorter when the double talk is not detected, it is possibleto fully converge the estimated errors when the double talk exists, andfurther, quicken the convergence of the estimated errors when there isno double talk.

An inventive echo cancel method is associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive method comprisesinputting a plurality of low-correlation audio signals which areobtained by suitably combining first audio signals of multi-channels andwhich have a lower correlation with each other than that among saidfirst audio signals of multi-channels, generating second audio signalsof multi-channels having a correlation with each other by computationbased on the inputted low-correlation audio signals, feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds, feeding the generated second audio signals orthe inputted low-correlation audio signals to filters, estimatingindividual transfer functions of said plurality of said audio transfersystems or a plurality of composite transfer functions obtained bysuitably combining said individual transfer functions based on theinputted low-correlation audio signals so as to set corresponding filtercharacteristics, producing echo cancel signals by applying said setfilter characteristics to the second audio signals or thelow-correlation audio signals fed to the filters, and subtracting saidecho cancel signals from collected audio signals obtained by collectingthe reproduced audio sounds by the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

Preferably, in the inventive echo cancel method, the inputtedlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

Another inventive echo cancel method is associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive method comprisesinputting a plurality of first low-correlation audio signals which areobtained by suitably combining first audio signals of multi-channels andwhich have a lower correlation with each other than that among saidfirst audio signals of multi-channels, generating second audio signalsof multi-channels having a correlation with each other by computationbased on the inputted first low-correlation audio signals, feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds, generating second low-correlation audio signalsof multi-channels based on the generated second audio signals, feedingthe generated second audio signals or the generated secondlow-correlation audio signals to filters, estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the generated secondlow-correlation audio signals so as to set corresponding filtercharacteristics, producing echo cancel signals by applying said setfilter characteristics to the second audio signals or the secondlow-correlation audio signals fed to the filters, and subtracting saidecho cancel signals from collected audio signals obtained by collectingthe reproduced audio sounds at the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

Preferably, in the inventive echo cancel method, the inputted firstlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

An inventive echo canceller is associated to a space provided thereinwith a plurality of loudspeakers and one or a plurality of microphonesfor forming a plurality of audio transfer systems through which audiosignals of multi-channels having a correlation with each other arereproduced by said respective loudspeakers and are collected by saidmicrophones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive echo cancellercomprises an inputting means for inputting a plurality oflow-correlation audio signals which are obtained by suitably combiningfirst audio signals of multi-channels and which have a lower correlationwith each other than that among said first audio signals ofmulti-channels, a demodulating means provided for generating secondaudio signals of multi-channels having a correlation with each other bydemodulating the inputted low-correlation audio signals, and for feedingthe generated second audio signals to the respective loudspeakers so asto reproduce audio sounds, an estimating means for estimating individualtransfer functions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the inputted low-correlationaudio signals so as to set corresponding filter characteristics, afilter means for producing echo cancel signals by applying said setfilter characteristics to the second audio signals or thelow-correlation audio signals fed to the filter means, and a subtractingmeans for subtracting said echo cancel signals from collected audiosignals obtained by collecting the reproduced audio sounds at themicrophones or from composite audio signals obtained by suitablycombining said collected audio signals, thereby performing the echocancellation.

Preferably, in the inventive echo canceller, the inputtedlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

Another inventive echo canceller is associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive echo cancellercomprises an inputting means for inputting a plurality of firstlow-correlation audio signals which are obtained by suitably combiningfirst audio signals of multi-channels and which have a lower correlationwith each other than that among said first audio signals ofmulti-channels, a demodulating means provided for generating secondaudio signals of multi-channels having a correlation with each other bydemodulating the inputted first low-correlation audio signals, and forfeeding the generated second audio signals to the respectiveloudspeakers so as to reproduce audio sounds, an estimating meansprovided for generating second low-correlation audio signals ofmulti-channels based on the generated second audio signals, and forestimating individual transfer functions of said plurality of said audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions based on thegenerated second low-correlation audio signals so as to setcorresponding filter characteristics, a filter means for producing echocancel signals by applying said set filter characteristics to the secondaudio signals or the second low-correlation audio signals fed to thefilter means, and a subtracting means for subtracting said echo cancelsignals from collected audio signals obtained by collecting thereproduced audio sounds at the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

Preferably, in the inventive echo canceller, the inputted firstlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

Preferably, in a inventive multi-channel echo canceller, themulti-channel audio signals being inputted from an outside and having acorrelation with each other are reproduced by said respectiveloudspeakers without lowering the correlation of the inputtedmulti-channel audio signals.

Preferably, the multi-channel audio signals being inputted from anoutside and having a correlation with each other are provisionallymodulated to lower the correlation, then demodulated to restore thecorrelation, and thereafter reproduced by said respective loudspeakers.Further, the multi-channel audio signals are provisionally modulated tolower the correlation by adding and subtracting the multi-channel audiosignals with each other, or by orthogonalizing the multi-channel audiosignals with each other.

In this invention, “audio signal” is not limited to human voices, butcovers all acoustic signals in the audible frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structural example in a stereo echocanceller 16, 24 of FIG. 2.

FIG. 2 is a block diagram showing an embodiment of a stereo soundtransfer apparatus of this invention.

FIG. 3 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 4 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 5 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 6 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 7 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 8 is a diagram showing the simulation measurement results of theecho cancel performance of the stereo echo canceller 16, 24 of FIG. 1.

FIG. 9 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 10 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 11 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 12 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 13 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 14 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 15 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 16 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 17 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 18 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 19 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 20 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 21 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 22 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 23 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 24 is a block diagram showing a modification of the structures ofFIGS. 1, 9 to 11.

FIG. 25 is a block diagram showing a modification of the structures ofFIGS. 12 to 15.

FIG. 26 is a block diagram showing a modification of the structures ofFIGS. 16 to 19.

FIG. 27 is a block diagram showing a modification of the structures ofFIGS. 20 and 21.

FIG. 28 is a block diagram showing another structural example in thestereo echo canceller 16, 24 of FIG. 2.

FIG. 29 is a time chart showing an example of unit intervals fororthogonalization processing and deriving an impulse response or itsestimated error in the stereo echo canceller of FIG. 28.

FIG. 30 is a diagram showing one example of functional blocks of anorthogonalizing filter 500 of FIG. 28.

FIG. 31 is a diagram showing one example of functional blocks oftransfer function calculating means 502 of FIG. 28.

FIG. 32 is a functional block diagram showing a modification of thetransfer function calculating means 502 of FIG. 31.

FIG. 33 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is no double talk.

FIG. 34 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is no double talk.

FIG. 35 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is no double talk.

FIG. 36 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is no double talk.

FIG. 37 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is double talk.

FIG. 38 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is double talk.

FIG. 39 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is double talk.

FIG. 40 is a diagram showing the simulation measurement result withrespect to a time-domain variation in echo cancellation amount of thestereo echo canceller 16, 24 of FIG. 28 when there is double talk.

FIG. 41 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

FIG. 42 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

FIG. 43 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

FIG. 44 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

FIG. 45 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is doubletalk.

FIG. 46 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is doubletalk.

FIG. 47 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is doubletalk.

FIG. 48 is a diagram showing the simulation measurement result withrespect to a time-domain variation in transfer function estimated errorof the stereo echo canceller 16, 24 of FIG. 28 when there is doubletalk.

FIG. 49 is an exemplary diagram for explaining an ensemble averagingprocess according to overlap processing.

FIG. 50 is a block diagram showing a modification of the stereo echocanceller 16, 24 of FIG. 28.

FIG. 51 is a block diagram showing a structural example wherein thenumber of microphones is modified to one in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinbelow. FIG.2 shows the whole structure of a two-way stereo sound transfer apparatusaccording to this invention. This is for performing two-way stereotransmission between a spot A and a spot B and, for example, isapplicable to the teleconferencing system. In the spot A, twoloudspeakers SP-A(L) and SP-A(R) and two microphones MC-A(L) and MC-A(R)are arranged in one space. Collected audio signals of the microphonesMC-A(L) and MC-A(R) are converted into digital signals at A/D converters12 and 14, respectively, and applied with echo cancel processing at astereo echo canceller 16, then modulated at a CODEC (CODER and DECODER)18 and transmitted to the spot B via a wire or radio transmission line20. In the spot B, two loudspeakers SP-B(L) and SP-B(R) and twomicrophones MC-B(L) and MC-B(R) are arranged in one space. Incidentally,assuming a situation that participants in the spots A and B talk witheach other in a face-to-face manner in the teleconferencing system,loudspeakers and microphones are arranged such that a sound collected bya microphone on the participant's left in one spot is reproduced from aloudspeaker on the participant's right in the other spot whereas a soundcollected by a microphone on the participant's right in one spot isreproduced from a loudspeaker on the participant's left in the otherspot. Specifically, when the loudspeaker SP-A(L) and the microphoneMC-A(L) are arranged on the participant's left and the loudspeakerSP-A(R) and the microphone MC-A(R) are arranged on the participant'sright in the spot A, the loudspeaker SP-B(L) and the microphone MC-B(L)are arranged on the participant's right and the loudspeaker SP-B(R) andthe microphone MC-B(R) are arranged on the participant's left in thespot B.

The signals transmitted from the spot A are inputted into a CODEC 22where the collected audio signals of the microphones MC-A(L) and MC-A(R)are demodulated. These demodulated collected audio signals of themicrophones MC-A(L) and MC-A(R) are respectively converted into analogsignals at D/A converters 26 and 28 via a stereo echo canceller 24, andrespectively reproduced at the loudspeakers SP-B(L) and SP-B(R).Collected audio signals of the microphones MC-B(L) and MC-B(R) in thespot B are converted into digital signals at A/D converters 30 and 32,respectively, and applied with echo cancel processing at the stereo echocanceller 24, then modulated at the CODEC 22 and transmitted to the spotA via the transmission line 20. The signals transmitted to the spot Aare inputted into the CODEC 18 where the collected audio signals of themicrophones MC-B(L) and MC-B(R) are demodulated. These demodulatedcollected audio signals of the microphones MC-B(L) and MC-B(R) arerespectively converted into analog signals at D/A converters 34 and 36via the stereo echo canceller 16, and respectively reproduced at theloudspeakers SP-A(L) and SP-A(R).

FIG. 1 shows a structural example in the stereo echo canceller 16, 24.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI (R) are outputted from sound output ends SO(L) and SO(R) as theyare (i.e. not through sum/difference signal producing means 52), andreproduced at loudspeakers SP(L) {representing SP-A(L) or SP-B(L)} andSP(R) {representing SP-A(R) or SP-B(R)}, respectively.

Filer means 40-1 to 40-4 are formed by, for example, FIR filters. Ofthem, the filter means 40-1 is set with an impulse responsecorresponding to a transfer function between the loudspeaker SP(L) and amicrophone MC(L) {representing MC-A(L) or MC-B(L)} and performs, usingsuch an impulse response, a convolution calculation of a signal to beoutputted from the sound output end SO(L), thereby producing an echocancel signal EC1 corresponding to a signal obtained such that thesignal outputted from the sound output end SO(L) is reproduced at theloudspeaker SP(L), collected by the microphone MC(L) and inputted into asound input end SI(L). The filter means 40-2 is set with an impulseresponse corresponding to a transfer function between the loudspeakerSP(L) and a microphone MC(R) {representing MC-A(R) or MC-B(R)} andperforms, using such an impulse response, a convolution calculation of asignal to be outputted from the sound output end SO(L), therebyproducing an echo cancel signal EC2 corresponding to a signal obtainedsuch that the signal outputted from the sound output end SO(L) isreproduced at the loudspeaker SP(L), collected by the microphone MC(R)and inputted into a sound input end SI(R). The filter means 40-3 is setwith an impulse response corresponding to a transfer function betweenthe loudspeaker SP(R) and the microphone MC(L) and performs, using suchan impulse response, a convolution calculation of a signal to beoutputted from the sound output end SO(R), thereby producing an echocancel signal EC3 corresponding to a signal obtained such that thesignal outputted from the sound output end SO(R) is reproduced at theloudspeaker SP(R), collected by the microphone MC(L) and inputted intothe sound input end SI(L). The filter means 40-4 is set with an impulseresponse corresponding to a transfer function between the loudspeakerSP(R) and the microphone MC(R) and performs, using such an impulseresponse, a convolution calculation of a signal to be outputted from thesound output end SO(R), thereby producing an echo cancel signal EC4corresponding to a signal obtained such that the signal outputted fromthe sound output end SO(R) is reproduced at the loudspeaker SP(R),collected by the microphone MC(R) and inputted into the sound input endSI (R).

An adder 44 performs a calculation of EC1+EC3. An adder 46 performs acalculation of EC2+EC4. A subtracter 48 subtracts an echo cancel signalEC1+EC3 from a collected audio signal of the microphone MC(L) inputtedfrom the sound input end SI(L), thereby to perform echo cancellation. Asubtracter 50 subtracts an echo cancel signal EC2+EC4 from a collectedaudio signal of the microphone MC(R) inputted from the sound input endSI(R), thereby to perform echo cancellation. These echo-canceled signalsof the respective left and right channels are outputted from line outputends LO(L) and LO(R), respectively, and transmitted toward the spot onthe counterpart side.

The sum/difference signal producing means 52 performs addition, using anadder 54, of the left/right two-channel stereo signals x_(L) and x_(R)inputted into the line input ends LI (L) and LI (R) so as to produce asum signal x_(L)+x_(R), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(L)−x_(R) (or itmay also be x_(R)−x_(L)). In case of the left/right two-channel stereosignals, the sum signal x_(L)+x_(R) and the difference signalx_(L)−x_(R) are in general low in correlation therebetween, andfrequently, approximately uncorrelated. Transfer function calculatingmeans 58 implements a cross-spectrum calculation between the sum signalx_(L)+x_(R) and the difference signal x_(L)−x_(R) produced by thesum/difference signal producing means 52 and signals e_(L) and e_(R)outputted from the subtracters 48 and 50 and, based on thiscross-spectrum calculation, sets filter characteristics (impulseresponses) of the filter means 40-1 to 40-4. Specifically, upon startingthe system, the filter characteristics of the filter means 40-1 to 40-4are not set, i.e. coefficients are all set to zero, so that the echocancel signals EC1 to EC4 are zero, and thus the collected audio signalsof the microphones MC(L) and MC(R) themselves are outputted from thesubtracters 48 and 50. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(L)+x_(R) and the difference signal x_(L)−x_(R) produced bythe sum/difference signal producing means 52 and the collected audiosignals e_(L) and e_(R) of the microphones MC(L) and MC(R) outputtedfrom the subtracters 48 and 50 and, based on this cross-spectrumcalculation, derives transfer functions of four audio transfer systemsbetween the loudspeakers SP(L) and SP(R) and the microphones MC(L) andMC(R), respectively, and implements initial setting of the filtercharacteristics of the filter means 40-1 to 40-4 to values correspondingto such transfer functions. After the initial setting, since the echocancel signals are produced by the filter means 40-1 to 40-4, the echocancel error signals e_(L) and e_(R) corresponding to difference signalsbetween the collected audio signals of the microphones MC(L) and MC(R)and the echo cancel signals EC1 to EC4 are outputted from thesubtracters 48 and 50. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(L)+x_(R) and the difference signal x_(L)−x_(R) produced bythe sum/difference signal producing means 52 and the echo cancel errorsignals e_(L) and e_(R) outputted from the subtracters 48 and 50 and,based on this cross-spectrum calculation, derives estimated errors ofthe transfer functions of the four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and updates the filter characteristics of the filter means40-1 to 40-4 to values that cancel the estimated errors, respectively.By repeating this updating operation per prescribed time period, theecho cancel error can be converged to a minimum value. Further, even ifthe transfer functions change due to movement of the microphonepositions or the like, the echo cancel error can be converged to aminimum value by sequentially updating the filter characteristics of thefilter means 40-1 to 40-4 depending thereon.

Correlation detecting means 60 detects a correlation between the sumsignal x_(L)+x_(R) and the difference signal x_(L)−x_(R) based on acorrelation value calculation or the like, and stops updating of theforegoing filter characteristics when the correlation value is no lessthan a prescribed value. When the correlation value becomes lower thanthe prescribed value, updating of the foregoing filter characteristicsis restarted. Incidentally, as a concrete technique for deriving thecorrelation between the sum signal x_(L)+x_(R) and the difference signalx_(L)−x_(R), any of the known techniques for detecting a correlation oftwo signals may be used.

Herein, the filter characteristics (impulse responses) that are set tothe filter means 40-1 to 40-4 by the transfer function calculating means58 will be described. In the following description, the transferfunctions and the filter characteristics are expressed using thefollowing symbols.

-   -   H_(xx): a transfer function (frequency-axis expression)    -   h_(xx): an impulse response (time-axis expression) corresponding        to H_(xx)    -   H^_(xx): an estimated transfer function (a transfer function set        to a filter)    -   h^_(xx): an impulse response corresponding to H^_(xx)    -   ΔH_(xx): a transfer function estimated error    -   Δh_(xx): an impulse response corresponding to ΔH_(xx)    -   (note: Suitable symbols are allocated to xx.)

The sum signal and the difference signal are respectively defined asfollows.sum signal: x _(M)(=x _(L) +x _(R))difference signal: x _(S)(=x _(L) −x _(R))

The transfer functions of the four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R) arerespectively defined as follows.

-   -   H_(LL): a transfer function of the system from the loudspeaker        SP(L) to the microphone MC(L)    -   H_(LR): a transfer function of the system from the loudspeaker        SP(L) to the microphone MC(R)    -   H_(RL): a transfer function of the system from the loudspeaker        SP(R) to the microphone MC(L)    -   H_(RR): a transfer function of the system from the loudspeaker        SP(R) to the microphone MC(R)

The input signals x_(L) and x_(R) at the line input ends LI(L) and LI(R)of the stereo echo canceller 16, 24 are replaced byx _(L)=(x _(M) +x _(S))/2x _(R)=(x _(M) −x _(S))/2.

Then, a calculation shown below is carried out in the transfer functioncalculating means 58. The signals e_(L) and e_(R) {the collected audiosignals of the microphones MC(L) and MC(R) as they are before theinitial setting of the filter means 40-1 to 40-4 whereas the echo cancelerror signals after the initial setting} outputted from the subtracters48 and 50, assuming that frequency-axis expressions of the signalsx_(M), x_(S), e_(L) and e_(R) are respectively given as X_(M), X_(S),E_(L) and E_(R), becomeE _(L)={(X _(M) +X _(S))H _(LL)/2}+{(X _(M) −X _(S))H _(RL)/2}E _(R)={(X _(M) +X _(S))H _(LR)/2}+{(X _(M) −X _(S))H _(RR)/2}hence

$\begin{matrix}\begin{matrix}{{2E_{L}} = {{\left( {X_{M} + X_{S}} \right)H_{LL}} + {\left( {X_{M} - X_{S}} \right)H_{RL}}}} \\{= {{X_{M}\left( {H_{LL} + H_{RL}} \right)} + {X_{S}\left( {H_{LL} - H_{RL}} \right)}}}\end{matrix} & (1) \\\begin{matrix}{{2E_{R}} = {{\left( {X_{M} + X_{S}} \right)H_{LR}} + {\left( {X_{M} - X_{S}} \right)H_{RR}}}} \\{= {{X_{M}\left( {H_{LR} + H_{RR}} \right)} + {{X_{S}\left( {H_{LR} - H_{RR}} \right)}.}}}\end{matrix} & (2)\end{matrix}$

When both sides of the equation (1) are multiplied by complex conjugatesX_(M)* and X_(S)* of X_(M) and X_(S) (i.e. deriving cross spectra) andensemble-averaged,ΣX _(M)*·2E _(L) =ΣX _(M) *·X _(M)(H _(LL) +H _(RL))+ΣX _(M) *·X _(S)(H_(LL) −H _(RL))  (3)ΣX _(S)*·2E _(L) =ΣX _(S) *·X _(M)(H _(LL) +H _(RL))+ΣX _(S) *·X _(S)(H_(LL) −H _(RL))  (4)are respectively obtained. Similarly, when both sides of the equation(2) are multiplied by complex conjugates X_(M)* and X_(S)* of X_(M) andX_(S),ΣX _(M)*·2E _(R) =ΣX _(M) *·X _(M)(H _(LR) +H _(RR))+ΣX _(M) *·X _(S)(H_(LR) −H _(RR))  (5)ΣX _(S)*·2E _(R) =ΣX _(S) *·X _(M)(H _(LR) +H _(RR))+ΣX _(S) *·X _(S)(H_(LR) −H _(RR))  (6)are respectively obtained.

In the equations (3) to (6), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (3) to (6) respectively becomeΣX _(M)*·2E _(L) =Σ|X _(M)|²(H _(LL) +H _(RL))  (3′)ΣX _(S)*·2E _(L) =Σ|X _(S)|²(H _(LL) −H _(RL))  (4′)ΣX _(M)*·2E _(R) =Σ|X _(M)|²(H _(LR) +H _(RR))  (5′)ΣX _(S)*·2E _(R) =Σ|X _(S)|²(H _(LR) −H _(RR))  (6′).

By transforming the equations (3′) to (6′), the following compositetransfer functions each in the form of combination of two transferfunctions are respectively derived.H _(LL) +H _(RL) =ΣX _(M)*·2E _(L) /Σ|X _(M)|²  (3″)H _(LL) −H _(RL) =ΣX _(S)*·2E _(L) /Σ|X _(S)|²  (4″)H _(LR) +H _(RR) =ΣX _(M)*·2E _(R) /Σ|X _(M)|²  (5″)H _(LR) −H _(RR) =ΣX _(S)*·2E _(R) /Σ|X _(S)|²  (6″)

When the corresponding sides of the equations (3″) and (4″) are addedtogether,H _(LL)=(ΣX _(M) *·E _(L) /Σ|X _(M)|²)+(ΣX _(S) *·E _(L) /Σ|X_(S)|²)  (7).

When subtraction is performed between the corresponding sides of theequations (3″) and (4″),H _(RL)=(ΣX _(M) *·E _(L) /Σ|X _(M)|²)+(ΣX _(S) *·E _(L) /Σ|X_(S)|²)  (8).

When the corresponding sides of the equations (5″) and (6″) are addedtogether,H _(LR)=(ΣX _(M) *·E _(R) /Σ|X _(M)|²)+(ΣX _(S) *·E _(R) /Σ|X_(S)|²)  (9).

When subtraction is performed between the corresponding sides of theequations (5″) and (6″),H _(RR)=(ΣX _(M) *·E _(R) /Σ|X _(M)|²)−(ΣX _(S) *·E _(R) /Σ|X_(S)|²)  (10).

Hence, transfer functions H_(LL), H_(RL), H_(LR) and H_(RR) arerespectively derived. Impulse responses h_(LL), h_(RL), h_(LR) andh_(RR) obtained by applying the inverse Fourier transformation to thesederived transfer functions are the filter characteristics to be set tothe filter means 40-1, 40-2, 40-3 and 40-4, respectively. Therefore, thetransfer function calculating means 58 derives the respective transferfunctions H_(LL), H_(RL), H_(LR) and H_(RR) from the equations (7) to(10) based on the sum signal x_(M), the difference signal x_(S), and theoutput signals e_(L) and e_(R) of the subtracters 48 and 50 that areinputted, derives the impulse responses h_(LL), h_(RL), h_(LR) andh_(RR) by applying the inverse Fourier transformation to those derivedtransfer functions, sets the derived impulse responses to the filtermeans 40-1, 40-2, 40-3 and 40-4 as h^_(LL), h^_(RL), h^_(LR) andh^_(RR), respectively, and further, updates the impulse responses byrepeating this calculation per suitably determined prescribed timeperiod (e.g. time period of performing ensemble averaging).

When the foregoing impulse response updating technique is explainedusing estimated error parameters, it becomes as follows. Output signals(collected audio signals) y_(L) and y_(R) of the microphones MC(L) andMC(R), assuming that frequency-axis expressions of x_(L), x_(R), y_(L)and y_(R) are respectively given as X_(L), X_(R), Y_(L) and Y_(R),becomeY _(L)=(X _(M) +X _(S))H _(LL)/2+(X _(M) −X _(S))H _(RL)/2Y _(R)=(X _(M) +X _(S))H _(LR)/2+(X _(M) −X _(S))H _(RR)/2.

The signal E_(L) outputted from the subtracter 48 becomes

$\begin{matrix}{E_{L} = {Y_{L} - \left( {{X_{L} \cdot H_{LL}^{\hat{}}} + {X_{R} \cdot H_{RL}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}} \right\} -}} \\{\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}^{\hat{}}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}^{\hat{}}/2}}} \right\}}\end{matrix}$hence2E _(L) =X _(M)(H _(LL) +H _(RL) −H^_(LL) −H^_(RL))+X _(S)(H _(LL) −H_(RL) −H^_(LL) +H^ _(RL))  (11).

The signal E_(R) outputted from the subtracter 50 becomes

$\begin{matrix}{E_{R} = {Y_{R} - \left( {{X_{L} \cdot H_{LR}^{\hat{}}} + {X_{R} \cdot H_{RR}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}} \right\} -}} \\{\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}^{\hat{}}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}^{\hat{}}/2}}} \right\}}\end{matrix}$hence2E _(R) =X _(M)(H _(LR) +H _(RR) −H^_(LR) −H^_(RR))+X _(S)(H _(LR) −H_(RR) −H^_(LR) +H^ _(RR))  (12).

When the estimated errors are given asΔH _(LL) =H _(LL) −H^_(LL)ΔH _(RL) =H _(RL) −H^_(RL)ΔH _(LR) =H _(LR) −H^_(LR)ΔH _(RR) =H _(RR) −H^_(RR)the equations (11) and (12) become2E _(L) =X _(M)(ΔH _(LL) +ΔH _(RL))+X _(S)(ΔH _(LL) −ΔH _(RL))  (11′)2E _(R) =X _(M)(ΔH _(LR) +ΔH _(RR))+X _(S)(ΔH _(LR) −ΔH _(RR))  (12′).

When both sides of the equation (11′) are multiplied by complexconjugates X_(M)* and X_(S)* of X_(M) and X_(S) (i.e. deriving crossspectra) and ensemble-averaged,ΣX _(M)*·2E _(L) =ΣX _(M) *·X _(M)(ΔH _(LL) +ΔH _(RL))+ΣX _(M) *·X_(S)(ΔH _(LL) −ΔH _(RL))  (13)ΣX _(S)*·2E _(L) =ΣX _(S) *·X _(M)(ΔH _(LL) +ΔH _(RL))+ΣX _(S) *·X_(S)(ΔH _(LL) −ΔH _(RL))  (14)are respectively obtained. Similarly, when both sides of the equation(12′) are multiplied by complex conjugates X_(M)* and X_(S)* of X_(M)and X_(S),ΣX _(M)*·2E _(R) =ΣX _(M) *·X _(M)(ΔH _(LR) +ΔH _(RR))+ΣX _(M) *·X_(S)(ΔH _(LL) −ΔH _(RR))  (15)ΣX _(S)*·2E _(R) =ΣX _(S) *·X _(M)(ΔH _(LR) +ΔH _(RR))+ΣX _(S) *·X_(S)(ΔH _(LL) −ΔH _(RR))  (16)are respectively obtained.

In the equations (13) to (16), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (13) to (16) respectively becomeΣX _(M)*·2E _(L) =Σ|X _(M)|²(ΔH _(LL) +ΔH _(RL))  (13′)ΣX _(S)*·2E _(L) =Σ|X _(S)|²(ΔH _(LL) −ΔH _(RL))  (14′)ΣX _(M)*·2E _(R) =Σ|X _(M)|²(ΔH _(LR) +ΔH _(RR))  (15′)ΣX _(S)*·2E _(R) =Σ|X _(S)|²(ΔH _(LR) −ΔH _(RR))  (16′).

From the equations (13′) to (16′),ΔH _(LL) =ΣX _(M) *·E _(L) /Σ|X _(M)|² +ΣX _(S) *·E _(L) /Σ|X_(S)|²  (17)ΔH _(RL) =ΣX _(M) *·E _(L) /Σ|X _(M)|² −ΣX _(S) *·E _(L) /Σ|X_(S)|²  (18)ΔH _(LR) =ΣX _(M) *·E _(R) /Σ|X _(M)|² +ΣX _(S) *·E _(R) /Σ|X_(S)|²  (19)ΔH _(RR) =ΣX _(M) *·E _(R) /Σ|X _(M)|² −ΣX _(S) *·E _(R) /Σ|X_(S)|²  (20)are respectively derived.

Using the estimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR) derivedfrom the equations (17) to (20), the filter characteristics of thefilter means 40-1, 40-2, 40-3 and 40-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(LL), h_(RL), h_(LR) and h_(RR) after K-th updating are given ash_(LL)(k), h_(RL)(k), h_(LR)(k) and h_(RR)(k), using impulse responsesΔh_(LL), ΔH_(RL), Δh_(LR) and Δh_(RR) corresponding to the derivedestimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR),h _(LL)(k+1)=h _(LL)(k)+αΔh _(LL)  (21)h _(RL)(k+1)=h _(RL)(k)+αΔh _(RL)  (22)h _(LR)(k+1)=h _(LR)(k)+αΔh _(LR)  (23)h _(RR)(k+1)=h _(RR)(k)+αΔh _(RR)  (24)where α is a suitably set convergence coefficient.

Using these updating equations, (k+1)th impulse responses h_(LL)(k+1),h_(RL)(k+1), h_(LR)(k+1) and h_(RR)(k+1) are derived and set to thefilter means 40-1, 40-2, 40-3 and 40-4, respectively, which is repeatedper suitably determined prescribed time period (e.g. time period ofperforming ensemble averaging).

The results of carrying out simulations using the signals x_(L) andx_(R) or the sum and difference signals x_(M) and x_(S) as referencesignals with respect to the stereo echo canceller 16, 24 of FIG. 1, areshown in FIGS. 3 to 8 for each of the audio transfer systems. As thesignals x_(L) and x_(R), stereo audio signals based on a human voicewere used. In FIGS. 3 to 8, the axis of abscissas represents the numberof blocks (one block represents a time period of performing ensembleaveraging and is set to about 2.3 seconds in the simulations), and thefilter characteristics are updated per block. The filter characteristicsare not set in the first block, the initial setting is executed in thesecond block, then updating is carried out per block. The axis ofordinates represents the estimated error (dB) of the transfer function,and the initial state where the filter characteristics are not set isdefined as 0 dB. Table 1 shows the conditions of the respectivesimulations of FIGS. 3 to 8.

TABLE 1 Reference Double Change in Figure Number Signal Talk TransferSystem FIG. 3 x_(L), x_(R) NO NO FIG. 4 x_(M), x_(S) NO NO FIG. 5 x_(L),x_(R) YES NO FIG. 6 x_(M), x_(S) YES NO FIG. 7 x_(L), x_(R) YES YES FIG.8 x_(M), x_(S) YES YES

In Table 1, “Double Talk” represents the state where sounds reproducedfrom the loudspeakers SP(L) and SP(R) as well as a voice uttered by aperson present in that room are simultaneously collected by themicrophones MC(L) and MC(R). Since the teleconferencing system is usedin general in the state where the double talk occurs, it is requiredthat the sufficient echo cancel performance can be achieved even whenthe double talk occurs. On the other hand, in Table 1, “Change inTransfer System” represents changing the transfer functions while theestimated errors are converging, supposing a case where the microphonepositions are moved, or the like. As an operation of the echo canceller,it is required that even if the estimated error temporarily increasesdue to change in transfer function, it again go toward convergence.

The simulation results of FIGS. 3 to 8 are considered. Comparing FIGS. 3and 4, when the signals x_(L) and x_(R) are used as reference signals(FIG. 3), because a correlation between the signals x_(L) and x_(R) ishigh, estimated errors only drop to about −15 to −25 dB at most in the20th block (about 45 seconds) from the start of the operation. Incontrast, when the signals x_(M) and x_(S) are used as reference signals(FIG. 4), because a correlation between the signals x_(M) and x_(S) islow, estimated errors drop to about −35 to −45 dB in the 20th block fromthe start of the operation, and thus it is seen that the sufficient echocancel performance can be achieved. Comparing FIGS. 5 and 6 showingcases where the double talk exists, when the signals x_(L) and x_(R) areused as reference signals (FIG. 5), estimated errors only drop to about−10 to −20 dB at most in the 20th block from the start of the operation.In contrast, when the signals x_(M) and x_(S) are used as referencesignals (FIG. 6), estimated errors drop to about −23 to −30 dB, and thusit is seen that even if the double talk exists, the sufficient echocancel performance can be achieved. This is because, since a voiceuttered by a person present in a room is uncorrelated with soundsreproduced from the loudspeakers SP(L) and SP(R), when the transferfunction calculating means 58 calculates transfer functions of therespective systems, components of the voice uttered by the person in theroom are canceled through the foregoing cross-spectrum calculation andensemble-average calculation, so that the transfer functions of therespective systems can be derived with no influence of the double talk.Comparing FIGS. 7 and 8 showing cases where the double talk and thechange of the transfer systems are present, when the signals x_(L) andx_(R) are used as reference signals (FIG. 7), convergence of estimatederrors is poor after giving a change to the transfer systems in the 11thblock so that the estimated errors only drop to about −5 to −15 dB atmost in the 20th block. In contrast, when the signals x_(M) and x_(S)are used as reference signals (FIG. 8), convergence of estimated errorsis excellent even after the change is given to the transfer systems inthe 11th block so that the estimated errors drop to about −17 to −26 dBin the 20th block, and thus it is seen that even if the double talk andthe change of the transfer systems exist, the sufficient echo cancelperformance can be achieved.

FIG. 9 shows another structural example in the stereo echo canceller 16,24 of FIG. 2, wherein sum/difference signal producing means is arrangedon transmission lines to loudspeakers. The same symbols are used withrespect to those portions common to the foregoing structure of FIG. 1.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI(R) are inputted into sum/difference signal producing means 52.The sum/difference signal producing means 52 performs addition of thestereo signals x_(L) and x_(R) using an adder 54 so as to produce a sumsignal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Stereo audio signal demodulating means62 performs addition of the sum and difference signals x_(M) and x_(S)using an adder 64, and further, gives thereto a coefficient ½ using acoefficient multiplier 66 to recover the original signal x_(L), whileperforms subtraction of the sum and difference signals x_(M) and x_(S)using a subtracter 68, and further, gives thereto a coefficient ½ usinga coefficient multiplier 70 to recover the original signal x_(R). Therecovered signals x_(L) and x_(R) are outputted from sound output endsSO(L) and SO(R) and reproduced at loudspeakers SP(L) and SP(R),respectively.

Transfer function calculating means 58 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 52 and signalse_(L) and e_(R) outputted from subtracters 48 and 50 and, based on thiscross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 40-1 to 40-4. Operations thereof are thesame as those described with respect to the structure of FIG. 1.Operations of the other portions are also the same as those describedwith respect to the structure of FIG. 1.

FIG. 10 shows another structural example in the stereo echo canceller16, 24 of FIG. 2, wherein transmission is implemented between the spotsA and B of FIG. 2 in the signal form of the sum signal x_(M) and thedifference signal x_(S), instead of the signal form of the stereosignals x_(L) and x_(R). The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 1 or 9. A sum signalx_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R) (or itmay also be x_(R)−x_(L))} transmitted from the spot on the counterpartside and inputted into line input ends LI(L) and LI(R) are inputted intostereo audio signal demodulating means 62. The stereo audio signaldemodulating means 62 performs addition of the sum and differencesignals x_(M) and x_(S) using an adder 64, and further, gives thereto acoefficient ½ using a coefficient multiplier 66 to recover the originalsignal x_(L), while performs subtraction of the sum and differencesignals x_(M) and x_(S) using a subtracter 68, and further, givesthereto a coefficient ½ using a coefficient multiplier 70 to recover theoriginal signal x_(R). The recovered signals x_(L) and x_(R) areoutputted from sound output ends SO(L) and SO(R) and reproduced atloudspeakers SP(L) and SP(R), respectively.

Transfer function calculating means 58 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and signals e_(L) ande_(R) outputted from subtracters 48 and 50 and, based on thiscross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 40-1 to 40-4. Operations thereof are thesame as those described with respect to the structure of FIG. 1 or 9.Sum/difference signal producing means 72 performs addition, using anadder 73, of the signals e_(L) and e_(R) outputted from the subtracters48 and 50 so as to produce a sum signal e_(M)(=e_(L)+e_(R)), whileperforms subtraction thereof using a subtracter 75 so as to produce adifference signal e_(S){=e_(L)−e_(R) (or it may also be e_(R)−e_(L))},then sends them toward the spot on the counterpart side. Operations ofthe other portions are the same as those described with respect to thestructure of FIG. 1 or 9.

FIG. 11 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 1, 9 or 10. A sumsignal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L)} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 62. The stereoaudio signal demodulating means 62 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 64, and further, givesthereto a coefficient ½ using a coefficient multiplier 66 to recover theoriginal signal x_(L), while performs subtraction of the sum anddifference signals x_(M) and x_(S) using a subtracter 68, and further,gives thereto a coefficient ½ using a coefficient multiplier 70 torecover the original signal x_(R). The recovered signals x_(L) and x_(R)are outputted from sound output ends SO(L) and SO(R) and reproduced atloudspeakers SP(L) and SP(R), respectively.

Sum/difference signal producing means 52 performs addition, using anadder 54, of the stereo signals x_(L) and x_(R) recovered by the stereoaudio signal demodulating means 62 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Transfer function calculating means 58implements a cross-spectrum calculation between the sum signal x_(M) andthe difference signal x_(S) produced by the sum/difference signalproducing means 52 and signals e_(L) and e_(R) outputted fromsubtracters 48 and 50 and, based on this cross-spectrum calculation,performs setting and updating of filter characteristics of filter means40-1 to 40-4. Operations thereof are the same as those described withrespect to the structure of FIG. 1, 9 or 10. Sum/difference signalproducing means 72 performs addition, using an adder 73, of the signalse_(L) and e_(R) outputted from the subtracters 48 and 50 so as toproduce a sum signal e_(M)(=e_(L)+e_(R)), while performs subtractionthereof using a subtracter 75 so as to produce a difference signale_(S){=e_(L)−e_(R) (or it may also be e_(R)−e_(L))}, then sends themtoward the spot on the counterpart side. Operations of the otherportions are the same as those described with respect to the structureof FIG. 1, 9 or 10.

FIG. 12 shows a structural example in the stereo echo canceller 16, 24.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI (R) are outputted from sound output ends SO(L) and SO(R) as theyare (i.e. not through sum/difference signal producing means 152), andreproduced at loudspeakers SP(L) and SP(R), respectively.

The sum/difference signal producing means 152 performs addition, usingan adder 154, of the left/right two-channel stereo signals x_(L) andx_(R) inputted into the line input ends LI(L) and LI(R) so as to producea sum signal x_(M)(=x_(L)+x_(R)), while performs subtraction thereofusing a subtracter 156 so as to produce a difference signalx_(S){=x_(L)−x_(R) (or x_(R)−x_(L))}.

Filer means 140-1 to 140-4 are formed by, for example, FIR filters.These filter means 140-1 to 140-4 are each set with an impulse responsecorresponding to a composite transfer function in the form ofcombination of transfer functions of suitable two systems among transferfunctions H_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfersystems between the loudspeakers SP(L) and SP(R) and microphones MC(L)and MC(R), respectively, and perform a convolution calculation of thesum and difference signals (low-correlation composite signals) usingsuch impulse responses, thereby producing echo cancel signals EC1 toEC4, respectively.

An adder 144 performs a calculation of EC1+EC3. An adder 146 performs acalculation of EC2+EC4. A subtracter 148 subtracts an echo cancel signalEC1+EC3 from a collected audio signal of the microphone MC(L) inputtedfrom a sound input end SI(L), thereby to perform echo cancellation. Asubtracter 150 subtracts an echo cancel signal EC2+EC4 from a collectedaudio signal of the microphone MC(R) inputted from a sound input endSI(R), thereby to perform echo cancellation. These echo-canceled signalsof the respective left and right channels are outputted from line outputends LO(L) and LO(R), respectively, and transmitted toward the spot onthe counterpart side.

Transfer function calculating means 158 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 152 and signalse_(L) and e_(R) outputted from the subtracters 148 and 150 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics (impulse responses) of the filter means 140-1 to 140-4.Specifically, upon starting the system, the filter characteristics ofthe filter means 140-1 to 140-4 are not set, i.e. coefficients are allset to zero, so that the echo cancel signals EC1 to EC4 are zero, andthus the collected audio signals of the microphones MC(L) and MC(R)themselves are outputted from the subtracters 148 and 150. Therefore, atthis time, the transfer function calculating means 158 performs thecross-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 152 and the collected audio signals e_(L) and e_(R) of themicrophones MC(L) and MC(R) outputted from the subtracters 148 and 150and, based on this cross-spectrum calculation, derives a plurality ofcomposite transfer functions each in the form of combination of transferfunctions of suitable two systems among transfer functions H_(LL),H_(LR), H_(RL) and H_(RR) of four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and implements initial setting of the filtercharacteristics of the filter means 140-1 to 140-4 to valuescorresponding to such composite transfer functions. After the initialsetting, since the echo cancel signals are produced by the filter means140-1 to 140-4, the echo cancel error signals e_(L) and e_(R)corresponding to difference signals between the collected audio signalsof the microphones MC(L) and MC(R) and the echo cancel signals EC1 toEC4 are outputted from the subtracters 148 and 150. Therefore, at thistime, the transfer function calculating means 158 performs thecross-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 152 and the echo cancel error signals e_(L) and e_(R) outputtedfrom the subtracters 148 and 150 and, based on this cross-spectrumcalculation, derives estimated errors of the foregoing compositetransfer functions, respectively, and updates the filter characteristicsof the filter means 140-1 to 140-4 to values that cancel such estimatederrors, respectively. By repeating this updating operation perprescribed time period, the echo cancel error can be converged to aminimum value. Further, even if the transfer functions change due tomovement of the microphone positions or the like, the echo cancel errorcan be converged to a minimum value by sequentially updating the filtercharacteristics of the filter means 140-1 to 140-4 depending thereon.

Correlation detecting means 160 detects a correlation between the sumsignal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

Herein, the filter characteristics (impulse responses) that are set tothe filter means 140-1 to 140-4 by the transfer function calculatingmeans 158 will be described. In the transfer function calculating means158, the following calculation is performed.

(In Case of Fixed Type Operation)

The signals x_(L) and x_(R) arex _(L)=(x _(M) +x _(S))/2x _(R)=(x _(M) −x _(S))/2hence, output signals Y_(L) and Y_(R) of the microphones MC(L) and MC(R)become

$\begin{matrix}\begin{matrix}{Y_{L} = {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}}} \\{= {{X_{M}\left\{ {\left( {H_{LL} + H_{RL}} \right)/2} \right\}} + {X_{S}\left\{ {\left( {H_{LL} - H_{RL}} \right)/2} \right\}}}}\end{matrix} & (25) \\\begin{matrix}{Y_{R} = {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}}} \\{= {{X_{M}\left\{ {\left( {H_{LR} + H_{RR}} \right)/2} \right\}} + {X_{S}{\left\{ {\left( {H_{LR} - H_{RR}} \right)/2} \right\}.}}}}\end{matrix} & (26)\end{matrix}$

When the composite transfer functions are given asH _(ML)=(H _(LL) +H _(RL))/2H _(SL)=(H _(LL) −H _(RL))/2H _(MR)=(H _(LR) +H _(RR))/2H _(SR)=(H _(LR) −H _(RR))/2the equations (25) and (26) respectively becomeY _(L) =X _(M) ·H _(ML) +X _(S) ·H _(SL)  (25′)Y _(R) =X _(M) ·H _(MR) +X _(S) ·H _(SR)  (26′).

When both sides of the equations (25′) and (26′) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M) *·Y _(L) =ΣX _(M) *·X _(M) ·H _(ML) +ΣX _(M) *·X _(S) ·H_(SL)  (27)ΣX _(S) *·Y _(L) =ΣX _(S) *·X _(M) ·H _(ML) +ΣX _(S) *·X _(S) ·H_(SL)  (28)ΣX _(M) *·Y _(R) =ΣX _(M) *·X _(M) ·H _(MR) +ΣX _(M) *·X _(S) ·H_(SR)  (29)ΣX _(S) *·Y _(R) =ΣX _(S) *·X _(M) ·H _(MR) +ΣX _(S) *·X _(S) ·H_(SR)  (30)are respectively obtained.

In the equations (27) to (30), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (27) to (30) respectively becomeΣX _(M) *·Y _(L) =Σ|X _(M)|² H _(ML)  (27′)ΣX _(S) *·Y _(L) =Σ|X _(S)|² H _(SL)  (28′)ΣX _(M) *·Y _(R) =Σ|X _(M)|² H _(MR)  (29′)ΣX _(S) *·Y _(R) =Σ|X _(S)|² H _(SR)  (30′).

From the equations (27′) to (30′),H _(ML) =ΣX _(M) *·Y _(L) /Σ|X _(M)|²  (31)H _(SL) =ΣX _(X) *·Y _(L) /Σ|X _(S)|²  (32)H _(MR) =ΣX _(M) *·Y _(R) /Σ|X _(M)|²  (33)H _(SR) =ΣX _(S) *·Y _(R) /Σ|X _(S)|²  (34)are respectively derived.

Impulse responses h_(ML), h_(SL), h_(MR) and h_(SR) obtained by applyingthe inverse Fourier transformation to these derived composite transferfunctions H_(ML), H_(SL), H_(MR) and H_(SR) are the filtercharacteristics to be set to the filter means 140-1, 140-2, 140-3 and140-4, respectively. Therefore, the transfer function calculating means158 derives the respective composite transfer functions H_(ML), H_(SL),H_(MR) and H_(SR) from the equations (31) to (34) based on the sumsignal x_(M), the difference signal x_(S), and output signals y_(L) andy_(R) of the microphones MC(L) and MC(R) that are inputted, derives theimpulse responses h_(ML), h_(SL), h_(MR) and h_(SR) by applying theinverse Fourier transformation to those derived composite transferfunctions, sets the derived impulse responses to the filter means 140-1,140-2, 140-3 and 140-4, respectively, and further, updates the impulseresponses by repeating this calculation per suitably determinedprescribed time period (e.g. time period of performing ensembleaveraging).

(In Case of Adaptive Type Operation)

Assuming that the filter characteristics set to the filter means 140-1,140-2, 140-3 and 140-4 are given as H^_(ML), H^_(SL), H^_(MR) andH^_(SR) (h^_(ML), h^_(SL), h^_(MR) and h^_(SR) when expressed in termsof the impulse responses), the signals e_(L) and e_(R) outputted fromthe subtracters 148 and 150 become

$\begin{matrix}\begin{matrix}{E_{L} = {Y_{L} - \left( {{X_{M} \cdot H_{ML}^{\hat{}}} + {X_{S} \cdot H_{SL}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}} \right\} -}} \\{\left( {{X_{M} \cdot H_{ML}^{\hat{}}} + {X_{S} \cdot H_{SL}^{\hat{}}}} \right)} \\{= {{X_{M}\left\{ {{\left( {H_{LL} + H_{RL}} \right)/2} - H_{ML}^{\hat{}}} \right\}} +}} \\{X_{S}\left\{ {{\left( {H_{LL} - H_{RL}} \right)/2} - H_{SL}^{\hat{}}} \right\}}\end{matrix} & (35) \\\begin{matrix}{E_{R} = {Y_{R} - \left( {{X_{M} \cdot H_{MR}^{\hat{}}} + {X_{S} \cdot H_{SR}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}} \right\} -}} \\{\left( {{X_{M} \cdot H_{MR}^{\hat{}}} + {X_{S} \cdot H_{SR}^{\hat{}}}} \right)} \\{= {{X_{M}\left\{ {{\left( {H_{LR} + H_{RR}} \right)/2} - H_{MR}^{\hat{}}} \right\}} +}} \\{X_{S}{\left\{ {{\left( {H_{LR} - H_{RR}} \right)/2} - H_{SR}^{\hat{}}} \right\}.}}\end{matrix} & (36)\end{matrix}$

When the composite transfer functions are given asH _(ML)=(H _(LL) +H _(RL))/2H _(SL)=(H _(LL) −H _(RL))/2H _(MR)=(H _(LR) +H _(RR))/2H _(SR)=(H _(LR) −H _(RR))/2the equations (35) and (36) respectively becomeE _(L) =X _(M)(H _(ML) −H^_(ML))+X _(S)(H _(SL) −H^_(SL))  (35′)E _(R) =X _(M)(H _(MR) −H^_(MR))+X _(S)(H _(SR) −H^_(SR))  (36′).

When the estimated errors of the composite transfer functions are givenasΔH _(ML) =H _(ML) −H^_(ML)ΔH _(SL) =H _(SL) −H^_(SL)ΔH _(MR) =H _(MR) −H^_(MR)ΔH _(SR) =H _(SR) −H^_(SR)the equations (35′) and (36′) respectively becomeE _(L) =X _(M) ·ΔH _(ML) +X _(S) ·ΔH _(SL)  (35″)E _(R) =X _(M) ·ΔH _(MR) +X _(S) ·ΔH _(SR)  (36″).

When both sides of the equations (35″) and (36″) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M) *·E _(L) =ΣX _(M) *·X _(M) ·ΔH _(ML) +ΣX _(M) *·X _(S) ·ΔH_(SL)  (37)ΣX _(S) *·E _(L) =ΣX _(S) *·X _(M) ·ΔH _(ML) +ΣX _(S) *·X _(S) ·ΔH_(SL)  (38)ΣX _(M) *·E _(R) =ΣX _(M) *·X _(M) ·ΔH _(MR) +ΣX _(M) *·X _(S) ·ΔH_(SR)  (39)ΣX _(S) *·E _(R) =ΣX _(S) *·X _(M) ·ΔH _(MR) +ΣX _(S) *·X _(S) ·ΔH_(SR)  (40)are respectively obtained.

In the equations (37) to (40), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (37) to (40) respectively becomeΣX _(M) *·E _(L) =Σ|X _(M)|² ΔH _(ML)  (37′)ΣX _(S) *·E _(L) =Σ|X _(S)|² ΔH _(SL)  (38′)ΣX _(M) *·E _(R) =Σ|X _(M)|² ΔH _(MR)  (39′)ΣX _(S) *·E _(R) =Σ|X _(S)|² ΔH _(SR)  (40′).

From the equations (37′) to (40′),ΔH _(ML) =ΣX _(M) *·E _(L) /Σ|X _(M)|²  (41)ΔH _(SL) =ΣX _(S) *·E _(L) /Σ|X _(S)|²  (42)ΔH _(MR) =ΣX _(M) *·E _(R) /Σ|X _(M)|²  (43)ΔH _(SR) =ΣX _(S) *·E _(R) /Σ|X _(S)|²  (44)are respectively derived.

Using the estimated errors ΔH_(ML), ΔH_(SL), ΔH_(MR) and ΔH_(SR) derivedfrom the equations (40) to (44), the filter characteristics of thefilter means 140-1, 140-2, 140-3 and 140-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(ML), h_(SL), h_(MR) and h_(SR) after K-th updating are given ash_(ML)(k), h_(SL)(k), h_(MR)(k) and h_(SR)(k), using impulse responsesΔh_(ML), Δh_(SL), Δh_(MR) and Δh_(SR) corresponding to the derivedestimated errors ΔH_(ML), ΔH_(SL), ΔH_(MR) and ΔH_(SR),h _(ML)(k+1)=h _(ML)(k)+αΔh _(ML)  (45)h _(SL)(k+1)=h _(SL)(k)+αΔh _(SL)  (46)h _(MR)(k+1)=h _(MR)(k)+αΔh _(MR)  (47)h _(SR)(k+1)=h _(SR)(k)+αΔh _(SR)  (48).

Using these updating equations, (k+1)th impulse responses h_(ML)(k+1),h_(SL)(k+1), h_(MR)(k+1) and h_(SR)(k+1) are derived and set to thefilter means 140-1, 140-2, 140-3 and 140-4, respectively, which isrepeated per suitably determined prescribed time period (e.g. timeperiod of performing ensemble averaging).

FIG. 13 shows another structural example in the stereo echo canceller16, 24 of FIG. 2, wherein sum/difference signal producing means isarranged on transmission lines to loudspeakers. The same symbols areused with respect to those portions common to the foregoing structure ofFIG. 12. Left/right two-channel stereo signals x_(L) and x_(R)transmitted from the spot on the counterpart side and inputted into lineinput ends LI(L) and LI(R) are inputted into sum/difference signalproducing means 152. The sum/difference signal producing means 152performs addition of the stereo signals x_(L) and x_(R) using an adder154 so as to produce a sum signal x_(M)(=x_(L)+x_(R)), while performssubtraction thereof using a subtracter 156 so as to produce a differencesignal x_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))}. Stereo audiosignal demodulating means 162 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient ½ using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient ½ using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

Transfer function calculating means 158 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 152 and signalse_(L) and e_(R) outputted from subtracters 148 and 150 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 140-1 to 140-4. Operations thereof arethe same as those described with respect to the structure of FIG. 12.Operations of the other portions are also the same as those describedwith respect to the structure of FIG. 12.

FIG. 14 shows another structural example in the stereo echo canceller16, 24 of FIG. 2, wherein transmission is implemented between the spotsA and B of FIG. 2 in the signal form of the sum signal x_(M) and thedifference signal x_(S), instead of the signal form of the stereosignals x_(L) and x_(R). The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 12 or 13. A sumsignal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 162. The stereoaudio signal demodulating means 162 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient ½ using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient ½ using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

Transfer function calculating means 158 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and signals e_(L) ande_(R) outputted from subtracters 148 and 150 and, based on thiscross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 140-1 to 140-4. Operations thereof arethe same as those described with respect to the structure of FIG. 12 or13. Sum/difference signal producing means 172 performs addition, usingan adder 173, of the signals e_(L) and e_(R) outputted from thesubtracters 148 and 150 so as to produce a sum signale_(M)(=e_(L)+e_(R)), while performs subtraction thereof using asubtracter 175 so as to produce a difference signal e_(S){=e_(L)−e_(R)(or it may also be e_(R)−e_(L))}, then sends them toward the spot on thecounterpart side. Operations of the other portions are the same as thosedescribed with respect to the structure of FIG. 12 or 13.

FIG. 15 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 12, 13 or 14. A sumsignal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 162. The stereoaudio signal demodulating means 162 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient ½ using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient ½ using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

Sum/difference signal producing means 152 performs addition, using anadder 154, of the stereo signals x_(L) and x_(R) recovered by the stereoaudio signal demodulating means 162 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 156 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Transfer function calculating means158 implements a cross-spectrum calculation between the sum signal x_(M)and the difference signal x_(S) produced by the sum/difference signalproducing means 152 and signals e_(L) and e_(R) outputted fromsubtracters 148 and 150 and, based on this cross-spectrum calculation,performs setting and updating of filter characteristics of filter means140-1 to 140-4. Operations thereof are the same as those described withrespect to the structure of FIG. 12, 13 or 14. Sum/difference signalproducing means 172 performs addition, using an adder 173, of thesignals e_(L) and e_(R) outputted from the subtracters 148 and 150 so asto produce a sum signal e_(M)(=e_(L)+e_(R)), while performs subtractionthereof using a subtracter 175 so as to produce a difference signale_(S){=e_(L)−e_(R) (or it may also be e_(R)−e_(L))}, then sends themtoward the spot on the counterpart side. Operations of the otherportions are the same as those described with respect to the structureof FIG. 12, 13 or 14.

FIG. 16 shows another structural example in the stereo echo canceller16, 24. A sum signal x_(M)(=x_(L)+x_(R)) and a difference signalx_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmitted from thespot on the counterpart side and inputted into line input ends LI(L) andLI(R) are inputted into stereo audio signal demodulating means 262. Thestereo audio signal demodulating means 262 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 264, and further,gives thereto a coefficient ½ using a coefficient multiplier 266 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 268, andfurther, gives thereto a coefficient ½ using a coefficient multiplier270 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

Collected audio signals y_(L) and y_(R) of microphones MC(L) and MC(R)are inputted into sum/difference signal producing means 272. Thesum/difference signal producing means 272 implements addition of themicrophone collected audio signals y_(L) and y_(R) using an adder 273 soas to produce a sum signal y_(M), while implements subtraction thereofusing a subtracter 275 so as to produce a difference signal y_(S).

Filer means 240-1 to 240-4 are formed by, for example, FIR filters.These filter means 240-1 to 240-4 are each set with an impulse responsecorresponding to a composite transfer function in the form ofcombination of transfer functions of suitable two systems among transferfunctions H_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfersystems between the loudspeakers SP(L) and SP(R) and microphones MC(L)and MC(R), respectively, and perform a convolution calculation of theleft/right two-channel stereo signals x_(L) and x_(R) using such impulseresponses, thereby producing echo cancel signals EC1 to EC4,respectively.

An adder 244 performs a calculation of EC1+EC3. An adder 246 performs acalculation of EC2+EC4. A subtracter 248 subtracts an echo cancel signalEC1+EC3 from the sum signal y_(M), thereby to perform echo cancellation.A subtracter 250 subtracts an echo cancel signal EC2+EC4 from thedifference signal y_(S), thereby to perform echo cancellation. Signalse_(M) and e_(S) outputted from the subtracters 248 and 250 are outputtedfrom line output ends LO(L) and LO(R), respectively, and transmittedtoward the spot on the counterpart side.

Transfer function calculating means 258 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and the signals e_(M)and e_(S) outputted from the subtracters 248 and 250 and, based on thiscross-spectrum calculation, performs setting and updating of filtercharacteristics (impulse responses) of the filter means 240-1 to 240-4.Specifically, upon starting the system, the filter characteristics ofthe filter means 240-1 to 240-4 are not set, i.e. coefficients are allset to zero, so that the echo cancel signals EC1 to EC4 are zero, andthus the sum signal y_(M) and the difference signal y_(S) outputted fromthe sum/difference signal producing means 272, as they are, areoutputted from the subtracters 248 and 250. Therefore, at this time, thetransfer function calculating means 258 performs the cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and the sum signaly_(M) and the difference signal y_(S) outputted from the subtracters 248and 250 and, based on this cross-spectrum calculation, derives aplurality of composite transfer functions each in the form ofcombination of transfer functions of suitable two systems among transferfunctions H_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfersystems between the loudspeakers SP(L) and SP(R) and the microphonesMC(L) and MC(R), respectively, and implements initial setting of thefilter characteristics of the filter means 240-1 to 240-4 to valuescorresponding to such composite transfer functions. After the initialsetting, since the echo cancel signals are produced by the filter means240-1 to 240-4, the echo cancel error signals e_(M) and e_(S)corresponding to difference signals between the sum signal y_(M) and thedifference signal y_(S) outputted from the sum/difference signalproducing means 272 and the echo cancel signals EC1 to EC4 are outputtedfrom the subtracters 248 and 250. Therefore, at this time, the transferfunction calculating means 258 performs the cross-spectrum calculationbetween the sum signal x_(M) and the difference signal x_(S) inputtedfrom the line input ends LI(L) and LI(R) and the echo cancel errorsignals e_(M) and e_(S) outputted from the subtracters 248 and 250 and,based on this cross-spectrum calculation, derives estimated errors ofthe foregoing composite transfer functions, respectively, and updatesthe filter characteristics of the filter means 240-1 to 240-4 to valuesthat cancel such estimated errors, respectively. By repeating thisupdating operation per prescribed time period, the echo cancel error canbe converged to a minimum value. Further, even if the transfer functionschange due to movement of the microphone positions or the like, the echocancel error can be converged to a minimum value by sequentiallyupdating the filter characteristics of the filter means 240-1 to 240-4depending thereon.

Correlation detecting means 260 detects a correlation between the sumsignal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

Herein, the filter characteristics (impulse responses) that are set tothe filter means 240-1 to 240-4 by the transfer function calculatingmeans 258 will be described. In the transfer function calculating means258, the following calculation is performed.

(In Case of Fixed Type Operation)

The sum and difference signals y_(M) and y_(S) of the output signalsy_(L) and y_(R) of the microphones MC(L) and MC(R), assuming thatfrequency-axis expressions of y_(M) and y_(S) are respectively given asy_(M) and y_(S), become

$\begin{matrix}\begin{matrix}{Y_{M} = {Y_{L} + Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) + \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}}}\end{matrix} & (49) \\\begin{matrix}{Y_{S} = {Y_{L} - Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) - \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} - H_{LR}} \right)} + {{X_{R}\left( {H_{RL} - H_{RR}} \right)}.}}}\end{matrix} & {(50).}\end{matrix}$

When the composite transfer functions are given asH _(LM) =H _(LL) +H _(LR)H _(RM) =H _(RL) +H _(RR)H _(LS) =H _(LL) −H _(LR)H _(RS) =H _(RL) −H _(RR)thenX _(L)=(X _(M) +X _(S))/2X _(R)=(X _(M) −X _(S))/2hence, the equations (49) and (50) respectively become

$\begin{matrix}{Y_{M} = {{\left( {X_{M} + X_{S}} \right){H_{LM}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RM}/2}}}} \\{= {{{X_{M}\left( {H_{LM} + H_{RM}} \right)}/2} + {{X_{S}\left( {H_{LM} - H_{RM}} \right)}/2}}} \\{Y_{S} = {{\left( {X_{M} + X_{S}} \right){H_{LS}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RS}/2}}}} \\{= {{{X_{M}\left( {H_{LS} + H_{RS}} \right)}/2} + {{X_{S}\left( {H_{LS} - H_{RS}} \right)}/2}}}\end{matrix}$thus2Y _(M) =X _(M)(H _(LM) +H _(RM))+X _(S)(H _(LM) −H _(RM))  (49′)2Y _(S) =X _(M)(H _(LS) +H _(RS))+X _(S)(H _(LS) −H _(RS))  (50′)

When both sides of the equations (49′) and (50′) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M) *·Y _(M) =ΣX _(M) *·X _(M)(H _(LM) +H _(RM))+ΣX _(M) *·X_(S)(H_(LM) −H _(RM))  (51)ΣX _(S) *·Y _(M) =ΣX _(S) *·X _(M)(H _(LM) +H _(RM))+ΣX _(S) *·X_(S)(H_(LM) −H _(RM))  (52)ΣX _(M) *·Y _(S) =ΣX _(M) *·X _(M)(H _(LS) +H _(RS))+ΣX _(M) *·X_(S)(H_(LS) −H _(RS))  (53)ΣX _(S) *·Y _(S) =ΣX _(S) *·X _(M)(H _(LS) +H _(RS))+ΣX _(M) *·X_(S)(H_(LS) −H _(RS))  (54)are respectively obtained.

In the equations (51) to (54), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (51) to (54) respectively becomeΣX _(M) *·Y _(M) =Σ|X _(M)|²(H _(LM) +H _(RM))  (51′)ΣX _(S) *·Y _(M) =Σ|X _(S)|²(H _(LM) −H _(RM))  (52′)ΣX _(M) *·Y _(S) =Σ|X _(M)|²(H _(LS) +H _(RS))  (53′)ΣX _(S) *·Y _(S) =Σ|X _(S)|²(H _(LS) −H _(RS))  (54′).

By transforming the equations (51′) to (54′), the following compositetransfer functions are respectively derived.H _(LM) +H _(RM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|²  (51″)H _(LM) −H _(RM) =ΣX _(S) *·Y _(M) /Σ|X _(S)|²  (52″)H _(LS) +H _(RS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|²  (53″)H _(LS) −H _(RS) =ΣX _(S) *·Y _(S) /Σ|X _(S)|²  (54″)

From the equations (51″) to (54″),H _(LM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|² +ΣX _(S) *·Y _(M) /Σ|X_(S)|²  (55)H _(RM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|² −ΣX _(S) *·Y _(M) /Σ|X_(S)|²  (56)H _(LS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|² +ΣX _(S) *·Y _(S) /Σ|X_(S)|²  (57)H _(RS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|² −ΣX _(S) *·Y _(S) /Σ|X_(S)|²  (58)are respectively derived.

Impulse responses h_(LM), h_(RM), h_(LS) and h_(RS) obtained by applyingthe inverse Fourier transformation to these derived composite transferfunctions H_(LM), H_(RM), H_(LS) and H_(RS) are the filtercharacteristics to be set to the filter means 240-1, 240-2, 240-3 and240-4, respectively. Therefore, the transfer function calculating means258 derives the respective composite transfer functions H_(LM), H_(RM),H_(LS) and H_(RS) from the equations (55) to (58) based on the sumsignal x_(M) and the difference signal x_(S) inputted into the lineinput ends LI(L) and LI(R) and the sum signal y_(M) and the differencesignal y_(S) outputted from the sum/difference signal producing means272, derives the impulse responses h_(LM), h_(RM), h_(LS) and h_(RS) byapplying the inverse Fourier transformation to those derived compositetransfer functions, sets the derived impulse responses to the filtermeans 240-1, 240-2, 240-3 and 240-4, respectively, and further, updatesthe impulse responses by repeating this calculation per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging).

(In Case of Adaptive Type Operation)

The signals e_(M) and e_(S) outputted from the subtracters 248 and 250,assuming that frequency-axis expressions of e_(M) and e_(S) arerespectively given as E_(M) and E_(S) and the filter characteristics setto the filter means 240-1, 240-2, 240-3 and 240-4 are given as H^_(LM),H^_(RM), H^_(LS) and H^_(RS) (h^_(LM), h^_(RM), h^_(LS) and h^_(RS) whenexpressed in terms of the impulse responses), become

$\begin{matrix}\begin{matrix}{E_{M} = {\left\{ {{L\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}} \right\} -}} \\{\left( {{X_{L} \cdot H_{LM}^{\hat{}}} + {X_{R} \cdot H_{RM}^{\hat{}}}} \right)} \\{= {{L\left\{ {\left( {H_{LL} + H_{LR}} \right) - H_{LM}^{\hat{}}} \right\}} + {X_{R}\left\{ {\left( {H_{RL} + H_{RR}} \right) - H_{RM}^{\hat{}}} \right\}}}}\end{matrix} & (59) \\\begin{matrix}{E_{S} = {\left\{ {{L\left( {H_{LL} - H_{LR}} \right)} + {X_{R}\left( {H_{RL} - H_{RR}} \right)}} \right\} -}} \\{\left( {{X_{L} \cdot H_{LS}^{\hat{}}} + {X_{R} \cdot H_{RS}^{\hat{}}}} \right)} \\{= {{L\left\{ {\left( {H_{LL} - H_{LR}} \right) - H_{LS}^{\hat{}}} \right\}} + {X_{R}\left\{ {\left( {H_{RL} - H_{RR}} \right) - H_{RS}^{\hat{}}} \right\}}}}\end{matrix} & (60)\end{matrix}$

When the composite transfer functions are given asH _(LM) =H _(LL) +H _(LR)H _(RM) =H _(RL) +H _(RR)H _(LS) =H _(LL) −H _(LR)H _(RS) =H _(RL) −H _(RR)thenX _(L)=(X _(M) +X _(S))/2X _(R)=(X _(M) −X _(S))/2hence, the equations (59) and (60) respectively becomeE _(M)=(X _(M) +X _(S))·(H _(LM) −H^_(LM))/2+(X _(M) −X _(S))·(H _(RM)−H^_(RM))/2  (61)E _(S)=(X _(M) +X _(S))·(H _(LS) −H^_(LS))/2+(X _(M) −X _(S))·(H _(RS)−H^_(RS))/2  (62).

When the estimated errors of the composite transfer functions are givenasΔH _(LM) =H _(LM) −H^_(LM)ΔH _(RM) =H _(RM) −H^_(RM)ΔH _(LS) =H _(LS) −H^_(LS)ΔH _(RS) =H _(RS) −H^_(RS)the equations (61) and (62) respectively becomeE _(M) =X _(M)(ΔH _(LM) +ΔH _(RM))/2+X _(S)(ΔH _(LM) −ΔH _(RM))/2E _(S) =X _(M)(ΔH _(LS) +ΔH _(RS))/2+X _(S)(ΔH _(LS) −ΔH _(RS))/2hence2E _(M) =X _(M)(ΔH _(LM) +ΔH _(RM))+X _(S)(ΔH _(LM) −ΔH _(RM))  (61′)2E _(S) =X _(M)(ΔH _(LS) +ΔH _(RS))+X _(S)(ΔH _(LS) −ΔH _(RS))  (62′).

When both sides of the equations (61′) and (62′) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M)*·2E _(M) =ΣX _(M) *·X _(M)(ΔH _(LM) +ΔH _(RM))+ΣX _(M) *·X_(S)(ΔH _(LM) −ΔH _(RM))  (63)ΣX _(S)*·2E _(M) =ΣX _(S) *·X _(M)(ΔH _(LM) +ΔH _(RM))+ΣX _(S) *·X_(S)(ΔH _(LM) −ΔH _(RM))  (64)ΣX _(M)*·2E _(S) =ΣX _(M) *·X _(M)(ΔH _(LS) +ΔH _(RS))+ΣX _(M) *·X_(S)(ΔH _(LS) −ΔH _(RS))  (65)ΣX _(S)*·2E _(S) =ΣX _(S) *·X _(M)(ΔH _(LS) +ΔH _(RS))+ΣX _(S) *·X_(S)(ΔH _(LS) −ΔH _(RS))  (66)are respectively obtained.

In the equations (63) to (66), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (63) to (66) respectively becomeΣX _(M)*·2E _(M) =Σ|X _(M)|²(ΔH _(LM) +H _(RM))  (63′)ΣX _(S)*·2E _(M) =Σ|X _(S)|²(ΔH _(LM) −H _(RM))  (64′)ΣX _(M)*·2E _(S) =Σ|X _(M)|²(ΔH _(LS) +H _(RS))  (65′)ΣX _(S)*·2E _(S) =Σ|X _(S)|²(ΔH _(LS) −H _(RS))  (66′).

By transforming the equations (63′) to (66′), the following compositetransfer functions are respectively derived.ΔH _(LM) +ΔH _(RM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|²  (63″)ΔH _(LM) −ΔH _(RM) =ΣX _(S)*·2E _(M) /Σ|X _(S)|²  (64″)ΔH _(LS) +ΔH _(RS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|²  (65″)ΔH _(LS) −ΔH _(RS) =ΣX _(S)*·2E _(S) /Σ|X _(S)|²  (66″)

From the equations (63″) to (66″),ΔH _(LM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|² +Σ _(S)*·2E _(M) /Σ|X_(S)|²  (67)ΔH _(RM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|² −Σ _(S)*·2E _(M) /Σ|X_(S)|²  (68)ΔH _(LS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|² +Σ _(S)*·2E _(S) /Σ|X_(S)|²  (69)ΔH _(RS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|² −Σ _(S)*·2E _(S) /Σ|X_(S)|²  (70)are respectively derived.

Using the estimated errors ΔH_(LM), ΔH_(RM), ΔH_(LS) and ΔH_(RS) derivedfrom the equations (67) to (70), the filter characteristics of thefilter means 240-1, 240-2, 240-3 and 240-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(LM), h_(RM), h_(LS) and h_(RS) after K-th updating are given ash_(LM)(k), h_(RM)(k), h_(LS)(k) and h_(RS)(k), using impulse responsesΔh_(LM), Δh_(RM), Δh_(LS) and Δh_(RS) corresponding to the derivedestimated errors ΔH_(LM), ΔH_(RM), ΔH_(LS) and ΔH_(RS),h _(LM)(k+1)=h _(LM)(k)+αΔh _(LM)  (71)h _(RM)(k+1)=h _(RM)(k)+αΔh _(RM)  (72)h _(LS)(k+1)=h _(LS)(k)+αΔh _(LS)  (73)h _(RS)(k+1)=h _(RS)(k)+αΔh _(RS)  (74).

Using these updating equations, (k+1)th impulse responses h_(LM)(k+1),h_(RM)(k+1), h_(LS)(k+1) and h_(RS)(k+1) are derived and set to thefilter means 240-1, 240-2, 240-3 and 240-4, respectively, which isrepeated per suitably determined prescribed time period (e.g. timeperiod of performing ensemble averaging).

FIG. 17 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 16. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through sum/difference signal producing means 252), and reproducedat loudspeakers SP(L) and SP(R), respectively. The sum/difference signalproducing means 252 performs addition of such stereo signals x_(L) andx_(R) using an adder 254 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 256 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}.

Transfer function calculating means 258 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 252 and signalse_(M) and e_(S) outputted from subtracters 248 and 250 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of the filter means 240-1 to 240-4. Operations thereofare the same as those described with respect to the structure of FIG.16. The signals e_(M) and e_(S) outputted from the subtracters 248 and250 are inputted into stereo audio signal demodulating means 282. Thestereo audio signal demodulating means 282 performs addition of thesignals e_(M) and e_(S) using an adder 284, and further, gives thereto acoefficient ½ using a coefficient multiplier 286 to recover aleft-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 288, and further, gives thereto acoefficient ½ using a coefficient multiplier 290 to recover aright-channel signal e_(R). The recovered signals e_(L) and e_(R) arerespectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 16.

FIG. 18 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 16 or 17. A sumsignal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 262. The stereoaudio signal demodulating means 262 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 264, and further,gives thereto a coefficient ½ using a coefficient multiplier 266 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 268, andfurther, gives thereto a coefficient ½ using a coefficient multiplier270 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively. Sum/differencesignal producing means 252 performs addition of such stereo signalsx_(L) and x_(R) using an adder 254 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 256 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}.

Transfer function calculating means 258 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 252 and signalse_(M) and e_(S) outputted from subtracters 248 and 250 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 240-1 to 240-4. Operations thereof arethe same as those described with respect to the structure of FIG. 16 or17. The signals e_(M) and e_(S) outputted from the subtracters 248 and250 are respectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 16 or 17.

FIG. 19 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 16, 17 or 18.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI(R) are inputted into sum/difference signal producing means 252.The sum/difference signal producing means 252 performs addition of thestereo signals x_(L) and x_(R) using an adder 254 so as to produce a sumsignal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 256 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. The produced sum signal x_(M) anddifference signal x_(S) are inputted into stereo audio signaldemodulating means 262. The stereo audio signal demodulating means 262performs addition of the sum and difference signals x_(M) and x_(S)using an adder 264, and further, gives thereto a coefficient ½ using acoefficient multiplier 266 to recover the original signal x_(L), whileperforms subtraction of the sum and difference signals x_(m) and x_(S)using a subtracter 268, and further, gives thereto a coefficient ½ usinga coefficient multiplier 270 to recover the original signal x_(R). Therecovered signals x_(L) and x_(R) are outputted from sound output endsSO(L) and SO(R) and reproduced at loudspeakers SP(L) and SP(R),respectively.

Transfer function calculating means 258 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 252 and signalse_(M) and e_(S) outputted from subtracters 248 and 250 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of the filter means 240-1 to 240-4. Operations thereofare the same as those described with respect to the structure of FIG.16, 17 or 18. The signals e_(M) and e_(S) outputted from the subtracters248 and 250 are inputted into stereo audio signal demodulating means282. The stereo audio signal demodulating means 282 performs addition ofthe signals e_(M) and e_(S) using an adder 284, and further, givesthereto a coefficient ½ using a coefficient multiplier 286 to recover aleft-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 288, and further, gives thereto acoefficient ½ using a coefficient multiplier 290 to recover aright-channel signal e_(R). The recovered signals e_(L) and e_(R) arerespectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 16, 17 or 18.

FIG. 20 shows another structural example in the stereo echo canceller16, 24. A sum signal x_(M)(=x_(L)+x_(R)) and a difference signalx_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmitted from thespot on the counterpart side and inputted into line input ends LI(L) andLI(R) are inputted into stereo audio signal demodulating means 362. Thestereo audio signal demodulating means 362 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 364, and further,gives thereto a coefficient ½ using a coefficient multiplier 366 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 368, andfurther, gives thereto a coefficient ½ using a coefficient multiplier370 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

Collected audio signals y_(L) and y_(R) of microphones MC(L) and MC(R)are inputted into sum/difference signal producing means 372. Thesum/difference signal producing means 372 implements addition of themicrophone collected audio signals y_(L) and y_(R) using an adder 373 soas to produce a sum signal y_(M), while implements subtraction thereofusing a subtracter 375 so as to produce a difference signal y_(S).

Filer means 340-1 to 340-4 are formed by, for example, FIR filters.These filter means 340-1 to 340-4 are each set with an impulse responsecorresponding to a composite transfer function in the form ofcombination of transfer functions H_(LL), H_(LR), H_(RL) and H_(RR) offour audio transfer systems between the loudspeakers SP(L) and SP(R) andmicrophones MC(L) and MC(R), respectively, and perform, using suchimpulse responses, a convolution calculation of the sum signal x_(M) andthe difference signal x_(S) inputted from the line input ends LI(L) andLI(R), thereby producing echo cancel signals EC1 to EC4, respectively.

An adder 344 performs a calculation of EC1+EC3. An adder 346 performs acalculation of EC2+EC4. A subtracter 348 subtracts an echo cancel signalEC1+EC3 from the sum signal y_(M), thereby to perform echo cancellation.A subtracter 350 subtracts an echo cancel signal EC2+EC4 from thedifference signal y_(S), thereby to perform echo cancellation. Signalse_(M) and e_(S) outputted from the subtracters 348 and 350 are outputtedfrom line output ends LO(L) and LO(R), respectively, and transmittedtoward the spot on the counterpart side.

Transfer function calculating means 358 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and the signals e_(M)and e_(S) outputted from the subtracters 348 and 350 and, based on thiscross-spectrum calculation, performs setting and updating of filtercharacteristics (impulse responses) of the filter means 340-1 to 340-4.Specifically, upon starting the system, the filter characteristics ofthe filter means 340-1 to 340-4 are not set, i.e. coefficients are allset to zero, so that the echo cancel signals EC1 to EC4 are zero, andthus the sum signal y_(M) and the difference signal y_(S) outputted fromthe sum/difference signal producing means 372, as they are, areoutputted from the subtracters 348 and 350. Therefore, at this time, thetransfer function calculating means 358 performs the cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)inputted from the line input ends LI(L) and LI(R) and the sum signale_(M) and the difference signal e_(S) outputted from the subtracters 348and 350 and, based on this cross-spectrum calculation, derives aplurality of composite transfer functions each in the form ofcombination of transfer functions H_(LL), H_(LR), H_(RL) and H_(RR) offour audio transfer systems between the loudspeakers SP(L) and SP(R) andthe microphones MC(L) and MC(R), respectively, and implements initialsetting of the filter characteristics of the filter means 340-1 to 340-4to values corresponding to such composite transfer functions. After theinitial setting, since the echo cancel signals are produced by thefilter means 340-1 to 340-4, the echo cancel error signals e_(M) ande_(S) corresponding to difference signals between the sum signal y_(M)and the difference signal y_(S) outputted from the sum/difference signalproducing means 372 and the echo cancel signals EC1 to EC4 are outputtedfrom the subtracters 348 and 350. Therefore, at this time, the transferfunction calculating means 358 performs the cross-spectrum calculationbetween the sum signal x_(M) and the difference signal x_(S) inputtedfrom the line input ends LI(L) and LI(R) and the echo cancel errorsignals e_(M) and e_(S) outputted from the subtracters 348 and 350 and,based on this cross-spectrum calculation, derives estimated errors ofthe foregoing composite transfer functions, respectively, and updatesthe filter characteristics of the filter means 340-1 to 340-4 to valuesthat cancel such estimated errors, respectively. By repeating thisupdating operation per prescribed time period, the echo cancel error canbe converged to a minimum value. Further, even if the transfer functionschange due to movement of the microphone positions or the like, the echocancel error can be converged to a minimum value by sequentiallyupdating the filter characteristics of the filter means 340-1 to 340-4depending thereon.

Correlation detecting means 360 detects a correlation between the sumsignal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

Herein, the filter characteristics (impulse responses) that are set tothe filter means 340-1 to 340-4 by the transfer function calculatingmeans 358 will be described. In the transfer function calculating means358, the following calculation is performed.

(In Case of Fixed Type Operation)

The sum and difference signals y_(M) and y_(S) of the output signalsy_(L) and y_(R) of the microphones MC(L) and MC(R) become

$\begin{matrix}\begin{matrix}{Y_{M} = {Y_{L} + Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) + \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}}}\end{matrix} & (71) \\\begin{matrix}{Y_{S} = {Y_{L} - Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) - \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} - H_{LR}} \right)} + {{X_{R}\left( {H_{RL} - H_{RR}} \right)}.}}}\end{matrix} & (72)\end{matrix}$X _(L)=(X _(M) +X _(S))/2X _(R)=(X _(M) −X _(S))/2hence, the equations (71) and (72) respectively become

$\begin{matrix}\begin{matrix}{Y_{M} = {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} + H_{LR}} \right)/2}} +}} \\{\left( {X_{M} - X_{S}} \right) \cdot {\left( {H_{RL} + H_{RR}} \right)/2}} \\{= {{{X_{M}\left( {H_{LL} + H_{LR} + H_{RL} + H_{RR}} \right)}/2} +}} \\{{X_{S}\left( {H_{LL} + H_{LR} - H_{RL} - H_{RR}} \right)}/2}\end{matrix} & \left( {71'} \right) \\\begin{matrix}{Y_{S} = {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} - H_{LR}} \right)/2}} +}} \\{\left( {X_{M} - X_{S}} \right) \cdot {\left( {H_{RL} - H_{RR}} \right)/2}} \\{= {{{X_{M}\left( {H_{LL} - H_{LR} + H_{RL} - H_{RR}} \right)}/2} +}} \\{{X_{S}\left( {H_{LL} - H_{LR} - H_{RL} + H_{RR}} \right)}/2}\end{matrix} & \left( {72'} \right)\end{matrix}$

When the composite transfer functions are given asH _(MM)=(H _(LL) +H _(LR) +H _(RL) +H _(RR))/2H _(SM)=(H _(LL) +H _(LR) −H _(RL) −H _(RR))/2H _(MS)=(H _(LL) −H _(LR) +H _(RL) −H _(RR))/2H _(SS)=(H _(LL) −H _(LR) −H _(RL) +H _(RR))/2the equations (71′) and (72′) respectively becomeY _(M) =X _(M) ·H _(MM) +X _(S) ·H _(SM)  (71″)Y _(S) =X _(M) ·H _(MS) +X _(S) /H _(SS)  (72″).

When both sides of the equations (71″) and (72″) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M) *·Y _(M) =ΣX _(M) *·X _(M) ·H _(MM) +ΣX _(M) *·X _(S) ·H_(SM)  (73)ΣX _(S) *·Y _(M) =ΣX _(S) *·X _(M) ·H _(MM) +ΣX _(S) *·X _(S) ·H_(SM)  (74)ΣX _(M) *·Y _(S) =ΣX _(M) *·X _(M) ·H _(MS) +ΣX _(M) *·X _(S) ·H_(SS)  (75)ΣX _(S) *·Y _(S) =ΣX _(M) *·X _(M) ·H _(MS) +ΣX _(S) *·X _(S) ·H_(SS)  (76)are respectively obtained.

In the equations (73) to (76), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (73) to (76) respectively becomeΣX _(M) *·Y _(M) =Σ|X _(M)|² ·H _(MM)  (73′)ΣX _(S) *·Y _(M) =Σ|X _(S)|² ·H _(SM)  (74′)ΣX _(M) *·Y _(S) =Σ|X _(M)|² ·H _(MS)  (75′)ΣX _(S) *·Y _(S) =Σ|X _(S)|² ·H _(SS)  (76′).

From the equations (73′) to (76′),H _(MM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|²  (77)H _(SM) =ΣX _(S) *·Y _(M) /Σ|X _(S)|²  (78)H _(MS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|²  (79)H _(SS) =ΣX _(S) *·Y _(S) /Σ|X _(S)|²  (80)are respectively derived.

Impulse responses h_(MM), h_(SM), h_(MS) and h_(SS) obtained by applyingthe inverse Fourier transformation to these derived composite transferfunctions H_(MM), H_(SM), H_(MS) and H_(SS) are the filtercharacteristics to be set to the filter means 340-1, 340-2, 340-3 and340-4, respectively. Therefore, the transfer function calculating means358 derives the respective composite transfer functions H_(MM), H_(SM),H_(MS) and H_(SS) from the equations (77) to (80) based on the sumsignal x_(M) and the difference signal x_(S) inputted into the lineinput ends LI(L) and LI(R) and the sum signal y_(M) and the differencesignal y_(S) outputted from the sum/difference signal producing means372, derives the impulse responses h_(MM), h_(SM), h_(MS) and h_(SS) byapplying the inverse Fourier transformation to those derived compositetransfer functions, sets the derived impulse responses to the filtermeans 340-1, 340-2, 340-3 and 340-4, respectively, and further, updatesthe impulse responses by repeating this calculation per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging).

(In Case of Adaptive Type Operation)

Assuming that the filter characteristics set to the filter means 340-1,340-2, 340-3 and 340-4 are given as H^_(MM), H^_(SM), H^_(MS) andH^_(SS) (h^_(MM), h^_(SM), h^_(MS) and h^_(SS) when expressed in termsof the impulse responses), the signals e_(M) and e_(S) outputted fromthe subtracters 348 and 350 become

$\begin{matrix}\begin{matrix}{E_{M} = \left\{ {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} + H_{LR}} \right)/2}} + {\left( {X_{M} - X_{S}} \right) \cdot}} \right.} \\{\left. {\left( {H_{RL} + H_{RR}} \right)/2} \right\} - \left( {{X_{M} \cdot H_{MM}^{\bigwedge}} + {H_{S} \cdot H_{SM}^{\bigwedge}}} \right)} \\{= {{X_{M}\left\lbrack {\left\{ {\left( {H_{LL} + H_{LR} + H_{RL} + H_{RR}} \right)/2} \right\} - H_{MM}^{\bigwedge}} \right\rbrack} +}} \\{X_{S}\left\lbrack {\left\{ {\left( {H_{{LL}\;} + H_{LR} - H_{RL} - H_{RR}} \right)/2} \right\} - H_{SM}^{\bigwedge}} \right\rbrack}\end{matrix} & (81) \\\begin{matrix}{E_{S} = \left\{ {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} - H_{LR}} \right)/2}} + {\left( {X_{M} - X_{S}} \right) \cdot}} \right.} \\{\left. {\left( {H_{RL} - H_{RR}} \right)/2} \right\} - \left( {{X_{M} \cdot H_{MS}^{\bigwedge}} + {X_{S} \cdot H_{SS}^{\bigwedge}}} \right)} \\{= {{X_{M}\left\lbrack {\left\{ {\left( {H_{LL} - H_{LR} + H_{RL} - H_{RR}} \right)/2} \right\} - H_{MS}^{\bigwedge}} \right\rbrack} +}} \\{X_{S}\left\lbrack {\left\{ {\left( {H_{{LL}\;} - H_{LR} - H_{RL} + H_{RR}} \right)/2} \right\} - H_{SS}^{\bigwedge}} \right\rbrack}\end{matrix} & (82)\end{matrix}$

When the composite transfer functions are given asH _(MM)=(H _(LL) +H _(LR) +H _(RL) +H _(RR))/2H _(SM)=(H _(LL) +H _(LR) −H _(RL) −H _(RR))/2H _(MS)=(H _(LL) −H _(LR) +H _(RL) −H _(RR))/2H _(SS)=(H _(LL) −H _(LR) −H _(RL) +H _(RR))/2the equations (81) and (82) respectively becomeE _(M) =X _(M)(H _(MM) −H^_(MM))+X _(S)(H _(SM) −H^_(SM))  (81′)E _(S) =X _(M)(H _(MS) −H^_(MS))+X _(S)(H _(SS) −H^_(SS))  (82′).

WhenΔH _(MM) =H _(MM) −H^_(MM)ΔH _(SM) =H _(SM) −H^_(SM)ΔH _(MS) =H _(MS) −H^_(MS)ΔH _(SS) =H _(SS) −H^_(SS)are given, the equations (81′) and (82′) respectively becomeE _(M) =X _(M) ·ΔH _(MM) +X _(S) ·ΔH _(SM)  (81″)E _(S) =X _(M) ·ΔH _(MS) +X _(S) ·ΔH _(SS)  (82″).

When both sides of the equations (81″) and (82″) are multiplied bycomplex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,ΣX _(M) *·E _(M) =ΣX _(M) *·X _(M) ·ΔH _(MM) +ΣX _(M) *·X _(S) ·ΔH_(SM)  (83)ΣX _(S) *·E _(M) =ΣX _(S) *·X _(M) ·ΔH _(MM) +ΣX _(S) *·X _(S) ·ΔH_(SM)  (84)ΣX _(M) *·E _(S) =ΣX _(M) *·X _(M) ·ΔH _(MS) +ΣX _(M) *·X _(S) ·ΔH_(SS)  (85)ΣX _(S) *·E _(S) =ΣX _(S) *·X _(M) ·ΔH _(MS) +ΣX _(S) *·X _(S) ·ΔH_(SS)  (86)are respectively obtained.

In the equations (83) to (86), since X_(M) and X_(S) are approximatelyuncorrelated with each other, such a term having X_(M)*·X_(S) orX_(S)*·X_(M) becomes approximately zero when ensemble-averaged. Further,X _(M) *·X _(M) =|X _(M)|²X _(S) *·X _(S) =|X _(S)|²hence, the equations (83) to (86) respectively becomeΣX _(M) *·E _(M) =Σ|X _(M)|² ·ΔH _(MM)  (83′)ΣX _(S) *·E _(M) =Σ|X _(S)|² ·ΔH _(SM)  (84′)ΣX _(M) *·E _(S) =Σ|X _(M)|² ·ΔH _(MS)  (85′)ΣX _(S) *·E _(S) =Σ|X _(S)|² ·ΔH _(SS)  (86′).

From the equations (83′) to (86′),ΔH _(MM) =ΣX _(M) *·E _(M) /Σ|X _(M)|²  (87)ΔH _(SM) =ΣX _(S) *·E _(M) /Σ|X _(S)|²  (88)ΔH _(MS) =ΣX _(M) *·E _(S) /Σ|X _(M)|²  (89)ΔH _(SS) =ΣX _(S) *·E _(S) /Σ|X _(S)|²  (90)are respectively derived.

Using the estimated errors ΔH_(MM), ΔH_(SM), ΔH_(MS) and ΔH_(SS) derivedfrom the equations (87) to (90), the filter characteristics of thefilter means 340-1, 340-2, 340-3 and 340-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(MM), h_(SM), h_(MS) and h_(SS) after K-th updating are given ash_(MM)(k), h_(SM)(k), h_(MS)(k) and h_(SS)(k), using impulse responsesΔh_(MM), Δh_(MS), Δh_(MS) and Δh_(SS) corresponding to the derivedestimated errors ΔH_(MM), ΔH_(SM), ΔH_(MS) and ΔH_(SS),h _(MM)(k+1)=h _(MM)(k)+αΔh _(MM)  (91)h _(SM)(k+1)=h _(SM)(k)+αΔh _(SM)  (92)h _(MS)(k+1)=h _(MS)(k)+αΔh _(MS)  (93)h _(SS)(k+1)=h _(SS)(k)+αΔh _(SS)  (94).

Using these updating equations, (k+1)th impulse responses h_(MM)(k+1),h_(SM)(k+1), h_(MS)(k+1) and h_(SS)(k+1) are derived and set to thefilter means 340-1, 340-2, 340-3 and 340-4, respectively, which isrepeated per suitably determined prescribed time period (e.g. timeperiod of performing ensemble averaging).

FIG. 21 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 20. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through sum/difference signal producing means 352), and reproducedat loudspeakers SP(L) and SP(R), respectively. The sum/difference signalproducing means 352 performs addition of such stereo signals x_(L) andx_(R) using an adder 354 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 356 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}.

Transfer function calculating means 358 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 352 and signalse_(M) and e_(S) outputted from subtracters 348 and 350 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of the filter means 340-1 to 340-4. Operations thereofare the same as those described with respect to the structure of FIG.20. The signals e_(M) and e_(S) outputted from the subtracters 348 and350 are inputted into stereo audio signal demodulating means 382. Thestereo audio signal demodulating means 382 performs addition of thesignals e_(M) and e_(S) using an adder 384, and further, gives thereto acoefficient ½ using a coefficient multiplier 386 to recover aleft-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 388, and further, gives thereto acoefficient ½ using a coefficient multiplier 390 to recover aright-channel signal e_(R). The recovered signals e_(L) and e_(R) arerespectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 20.

FIG. 22 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 20 or 21. A sumsignal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 362. The stereoaudio signal demodulating means 362 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 364, and further,gives thereto a coefficient ½ using a coefficient multiplier 366 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 368, andfurther, gives thereto a coefficient ½ using a coefficient multiplier370 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively. Sum/differencesignal producing means 352 performs addition of such stereo signalsx_(L) and x_(R) using an adder 354 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 356 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}.

Transfer function calculating means 358 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 352 and signalse_(M) and e_(S) outputted from subtracters 348 and 350 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of filter means 340-1 to 340-4. Operations thereof arethe same as those described with respect to the structure of FIG. 20 or21. The signals e_(M) and e_(S) outputted from the subtracters 348 and350 are respectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 20 or 21.

FIG. 23 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 20, 21 or 22.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI(R) are inputted into sum/difference signal producing means 352.The sum/difference signal producing means 352 performs addition of thestereo signals x_(L) and x_(R) using an adder 354 so as to produce a sumsignal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 356 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. The produced sum signal x_(M) anddifference signal x_(S) are inputted into stereo audio signaldemodulating means 362. The stereo audio signal demodulating means 362performs addition of the sum and difference signals x_(M) and x_(S)using an adder 364, and further, gives thereto a coefficient ½ using acoefficient multiplier 366 to recover the original signal x_(L), whileperforms subtraction of the sum and difference signals x_(M) and x_(S)using a subtracter 368, and further, gives thereto a coefficient ½ usinga coefficient multiplier 370 to recover the original signal x_(R). Therecovered signals x_(L) and x_(R) are outputted from sound output endsSO(L) and SO(R) and reproduced at loudspeakers SP(L) and SP(R),respectively.

Transfer function calculating means 358 implements a cross-spectrumcalculation between the sum signal x_(M) and the difference signal x_(S)produced by the sum/difference signal producing means 352 and signalse_(M) and e_(S) outputted from subtracters 348 and 350 and, based onthis cross-spectrum calculation, performs setting and updating of filtercharacteristics of the filter means 340-1 to 340-4. Operations thereofare the same as those described with respect to the structure of FIG.20, 21 or 22. The signals e_(M) and e_(S) outputted from the subtracters348 and 350 are inputted into stereo audio signal demodulating means382. The stereo audio signal demodulating means 382 performs addition ofthe signals e_(M) and e_(S) using an adder 384, and further, givesthereto a coefficient ½ using a coefficient multiplier 386 to recover aleft-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 388, and further, gives thereto acoefficient ½ using a coefficient multiplier 390 to recover aright-channel signal e_(R). The recovered signals e_(L) and e_(R) arerespectively outputted from line output ends LO(L) and LO(R) andtransmitted toward the spot on the counterpart side. Operations of theother portions are the same as those described with respect to thestructure of FIG. 20, 21 or 22.

This invention can take various structures in addition to the structuresas shown in the foregoing embodiments. For example, part of thestructures of FIGS. 1, 9 to 11 can be changed like FIG. 24.Specifically, sum/difference signal producing means 402 is disposedbetween the adders 44 and 46 and the subtracters 48 and 50, so as toperform addition of the echo cancel signals EC1+EC3 and EC2+EC4 using anadder 404 to produce an echo cancel signal (EC1+EC3)+(EC2+EC4), whileperform subtraction thereof using a subtracter 406 to produce(EC1+EC3)−(EC2+EC4). Further, sum/difference signal producing means 408is disposed between the sound input ends SI(L) and SI(R) and thesubtracters 48 and 50, so as to perform addition of the collected audiosignals y_(L) and y_(R) of the microphones MC(L) and MC(R) using anadder 410 to produce a sum signal y_(M), while perform subtractionthereof using a subtracter 412 to produce a difference signal y_(S). Thesubtracter 48 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) fromthe sum signal y_(M) to implement echo cancellation. The subtracter 50subtracts the echo cancel signal (EC1+EC3)−(EC2+EC4) from the differencesignal y_(S) to implement echo cancellation. The signals e_(M) and e_(S)outputted from the subtracters 48 and 50 are inputted into stereo audiosignal demodulating means 414. The stereo audio signal demodulatingmeans 414 performs addition of the signals e_(M) and e_(S) using anadder 416, and further, gives thereto a coefficient ½ using acoefficient multiplier 418 to recover a left-channel signal e_(L), whileperforms subtraction of the signals e_(M) and e_(S) using a subtracter420, and further, gives thereto a coefficient ½ using a coefficientmultiplier 422 to recover a right-channel signal e_(R). The otherportions are the same as those described with respect to the structuresof FIGS. 1, 9 to 11.

On the other hand, part of the structures of FIGS. 12 to 15 can bechanged like FIG. 25. Specifically, sum/difference signal producingmeans 432 is disposed between the adders 144 and 146 and the subtracters148 and 150, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 434 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 436 to produce (EC1+EC3)−(EC2+EC4). Further, sum/differencesignal producing means 438 is disposed between the sound input endsSI(L) and SI(R) and the subtracters 148 and 150, so as to performaddition of the collected audio signals y_(L) and y_(R) of themicrophones MC(L) and MC(R) using an adder 440 to produce a sum signaly_(M), while perform subtraction thereof using a subtracter 442 toproduce a difference signal y_(S). The subtracter 148 subtracts the echocancel signal (EC1+EC3)+(EC2+EC4) from the sum signal y_(M) to implementecho cancellation. The subtracter 150 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the difference signal y_(S) to implement echocancellation. The signals e_(M) and e_(S) outputted from the subtracters148 and 150 are inputted into stereo audio signal demodulating means444. The stereo audio signal demodulating means 444 performs addition ofthe signals e_(M) and e_(S) using an adder 446, and further, givesthereto a coefficient ½ using a coefficient multiplier 448 to recover aleft-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 450, and further, gives thereto acoefficient ½ using a coefficient multiplier 452 to recover aright-channel signal e_(R). The other portions are the same as thosedescribed with respect to the structures of FIGS. 12 to 15.

On the other hand, part of the structures of FIGS. 16 to 19 can bechanged like FIG. 26. Specifically, sum/difference signal producingmeans 462 is disposed between the adders 244 and 246 and the subtracters248 and 250, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 464 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 466 to produce (EC1+EC3)−(EC2+EC4). The sum/difference signalproducing means 272 of FIGS. 16 to 19 is not required. The subtracter248 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) from thecollected audio signal y_(L) of the microphone MC(L) to implement echocancellation. The subtracter 250 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the collected audio signal y_(R) of themicrophone MC(R) to implement echo cancellation. Sum/difference signalproducing means 468 is disposed on the output side of the subtracters248 and 250, so as to perform addition of the output signals e_(L) ande_(R) of the subtracters 248 and 250 using an adder 470 to produce a sumsignal e_(M), while perform subtraction thereof using a subtracter 472to produce a difference signal e_(S). The other portions are the same asthose described with respect to the structures of FIGS. 16 to 19.

On the other hand, part of the structures of FIGS. 20 and 21 can bechanged like FIG. 27. Specifically, sum/difference signal producingmeans 482 is disposed between the adders 344 and 346 and the subtracters348 and 350, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 484 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 486 to produce (EC1+EC3)−(EC2+EC4). The sum/difference signalproducing means 372 of FIGS. 20 to 23 is not required. The subtracter348 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) from thecollected audio signal y_(L) of the microphone MC(L) to implement echocancellation. The subtracter 350 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the collected audio signal y_(R) of themicrophone MC(R) to implement echo cancellation. Sum/difference signalproducing means 488 is disposed on the output side of the subtracters348 and 350, so as to perform addition of the output signals e_(L) ande_(R) of the subtracters 348 and 350 using an adder 490 to produce a sumsignal e_(M), while perform subtraction thereof using a subtracter 492to produce a difference signal e_(S). The other portions are the same asthose described with respect to the structures of FIGS. 20 to 23.

FIG. 28 shows another structural example in the stereo echo canceller16, 24 of FIG. 2. The same symbols are used with respect to thoseportions common to the foregoing structure of FIG. 1. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through an orthogonalizing filter 500), and reproduced atloudspeakers SP(L) and SP(R), respectively.

Filter means 40-1 is set with an impulse response corresponding to atransfer function between the loudspeaker SP(L) and a microphone MC(L),filter means 40-2 is set with an impulse response corresponding to atransfer function between the loudspeaker SP(L) and a microphone MC(R),filter means 40-3 is set with an impulse response corresponding to atransfer function between the loudspeaker SP(R) and the microphoneMC(L), and filter means 40-4 is set with an impulse responsecorresponding to a transfer function between the loudspeaker SP(R) andthe microphone MC(R).

The orthogonalizing filter 500 performs a principal component analysiswith respect to the input stereo signals x_(L) and x_(R) per prescribedtime period, and converts such input stereo signals x_(L) and x_(R) intotwo signals that are orthogonal to each other. Transfer functioncalculating means 502 implements a cross-spectrum calculation betweenthe mutually orthogonal two signals produced at the orthogonalizingfilter 500 and signals e_(L) and e_(R) outputted from subtracters 48 and50 and, based on this cross-spectrum calculation, sets filtercharacteristics (impulse responses) of the filter means 40-1 to 40-4.Specifically, upon starting the system, the filter characteristics ofthe filter means 40-1 to 40-4 are not set, i.e. coefficients are all setto zero, so that echo cancel signals EC1 to EC4 are zero, and thuscollected audio signals of the microphones MC(L) and MC(R) themselvesare outputted from the subtracters 48 and 50. Therefore, at this time,the transfer function calculating means 502 performs the cross-spectrumcalculation between the mutually orthogonal two signals produced at theorthogonalizing filter 500 and the collected audio signals e_(L) ande_(R) of the microphones MC(L) and MC(R) outputted from the subtracters48 and 50 and, based on this cross-spectrum calculation, derivestransfer functions of four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and implements initial setting of the filtercharacteristics of the filter means 40-1 to 40-4 to values correspondingto such transfer functions. After the initial setting, since the echocancel signals are produced by the filter means 40-1 to 40-4, the echocancel error signals e_(L) and e_(R) corresponding to difference signalsbetween the collected audio signals of the microphones MC(L) and MC(R)and the echo cancel signals EC1 to EC4 are outputted from thesubtracters 48 and 50. Therefore, at this time, the transfer functioncalculating means 502 performs the cross-spectrum calculation betweenthe mutually orthogonal two signals produced at the orthogonalizingfilter 500 and the echo cancel error signals e_(L) and e_(R) outputtedfrom the subtracters 48 and 50 and, based on this cross-spectrumcalculation, derives estimated errors of the transfer functions of thefour audio transfer systems between the loudspeakers SP(L) and SP(R) andthe microphones MC(L) and MC(R), respectively, and updates the filtercharacteristics of the filter means 40-1 to 40-4 to values that cancelthe estimated errors, respectively. By repeating this updating operationper prescribed time period, the echo cancel error can be converged to aminimum value. Further, even if the transfer functions change due tomovement of the microphone positions or the like, the echo cancel errorcan be converged to a minimum value by sequentially updating the filtercharacteristics of the filter means 40-1 to 40-4 depending thereon.

According to the known technique based on comparison between theleft/right two-channel stereo signals x_(L) and x_(R) inputted into theline input ends LI(L) and LI(R) and the signals e_(L) and e_(R) to beoutputted from line output ends LO(L) and LO(R), double talk detectingmeans 504 detects the double talk where sounds other than thosereproduced by the loudspeakers SP(L) and SP(R) are inputted into themicrophones MC(L) and MC(R). The transfer function calculating means 502makes relatively longer an update period of the filter characteristicsof the filter means 40-1 to 40-4 while the double talk is detected,whereas makes relatively shorter the update period of the filtercharacteristics while the double talk is not detected. This makes itpossible to fully converge estimated errors when the double talk exists,and further, quicken convergence of estimated errors when there is nodouble talk.

An orthogonalization process of the orthogonalizing filter 500 will bedescribed. The orthogonalization process is implemented per prescribedtime period of the input stereo signals. Herein, the orthogonalizationprocess is carried out per frame (e.g. 512 samples) as shown in FIG. 29(however, overlap processing may be implemented as shown in FIG. 49which will be described later). A vector having as its elements a samplegroup of one frame of the left-channel input signal x_(L) inputted intothe orthogonalizing filter 500, and a vector having as its elements asample group of one frame of the right-channel input signal x_(R) aregiven asx _(L) =(x _(L1) , x _(L2) , x _(L3) , . . . , x _(Ln))x _(R) =(x _(R1) , x _(R2) , x _(R3) , . . . , x _(Rn))  [Equation 1]

Since x_(L) and x_(R) are the stereo signals, they mutually have acorrelation. The orthogonalization process is carried out by performinga principal component analysis, using x_(L) and x_(R) as two variables,with respect to a sample group composed of combinations of such twovariables per frame so as to derive eigenvectors of the first principalcomponent and the second principal component that are mutuallyorthogonal, and projecting the respective samples composed ofcombinations of such two variables onto the derived eigenvectors of thefirst principal component and the second principal component,respectively.

Concrete contents of calculation of the orthogonalization process willbe described. Now, assuming that an observation matrix B is given as

$\begin{matrix}{B = \begin{pmatrix}{x_{L1},x_{L2},x_{L3},\ldots\mspace{11mu},x_{L\; n}} \\{x_{R1},x_{R2},x_{R3},\ldots\mspace{11mu},x_{R\; n}}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{20mu} 2} \right\rbrack\end{matrix}$then, a covariance matrix S of B becomes

$\begin{matrix}\begin{matrix}{S = {\frac{1}{n - 1}{BB}^{T}\mspace{31mu}\left( {B^{T}\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{transposed}\mspace{14mu}{matrix}\mspace{14mu}{of}\mspace{14mu} B} \right)}} \\{= {\frac{1}{n - 1}\begin{pmatrix}{x_{L1},x_{L2},x_{L3},\ldots\mspace{11mu},x_{L\; n}} \\{x_{R1},x_{R2},x_{R3},\ldots\mspace{11mu},x_{R\; n}}\end{pmatrix}\begin{pmatrix}x_{L1} & x_{R1} \\x_{L2} & x_{R2} \\. & . \\. & . \\x_{L\; n} & x_{Rn}\end{pmatrix}}} \\{= {\frac{1}{n - 1}\begin{pmatrix}{\sum\limits_{k = 1}^{n}\; x_{Lk}^{2}} & {\sum\limits_{k = 1}^{n}\;{x_{Lk}x_{Rk}}} \\{\sum\limits_{k = 1}^{n}\;{x_{Lk}x_{Rk}}} & {\sum\limits_{k = 1}^{n}\; x_{Rk}^{2}}\end{pmatrix}}} \\{= \begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}}\end{matrix} & \left\lbrack {{Equation}\mspace{20mu} 3} \right\rbrack\end{matrix}$

(S₁₁ is variance of x_(L), S₂₂ is variance of x_(R), and

S₁₂(=S₂₁) is covariance of x_(L) and x_(R))

Hence,

from

$\begin{matrix}{\begin{pmatrix}{S_{11} - \lambda} & S_{12} \\S_{21} & {S_{22} - \lambda}\end{pmatrix} = 0} & \left\lbrack {{Equation}\mspace{20mu} 4} \right\rbrack\end{matrix}$(S ₁₁−λ)(S ₂₂−λ)−S ₁₂ S ₂₁=0  [Equation 5]

is solved,

thus

$\begin{matrix}{\lambda = \frac{\begin{matrix}{S_{11} + {S_{22} \pm}} \\\sqrt{\left( {S_{11} + S_{22}} \right)^{2} - {4\left( {{S_{11}S_{22}} - S_{12}^{2}} \right)}}\end{matrix}}{2}} & \left\lbrack {{Equation}\mspace{20mu} 6} \right\rbrack\end{matrix}$so that two solutions for eigenvalues λ are derived.

Assuming that one of the two eigenvalues having greater variance(eigenvalue of the first principal component) is λ₁, an eigenvectorU_(max) corresponding to the eigenvalue λ₁ is

$\begin{matrix}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix} & \left\lbrack {{Equation}\mspace{20mu} 8} \right\rbrack\end{matrix}$which establishes

$\begin{matrix}{{\begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}} = {\lambda_{1}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}}} & \left\lbrack {{Equation}\mspace{20mu} 7} \right\rbrack\end{matrix}$

where u₁₁ ²+u₁₂ ²=1

By solving u₁₁ and u₁₂,

$\begin{matrix}{{u_{11} = {\pm \frac{S_{12}}{\sqrt{S_{12}^{2} - \left( {\lambda_{1} - S_{11}} \right)^{2}}}}}{u_{12} = {\pm \frac{\lambda_{1} - S_{11}}{\sqrt{S_{12}^{2} - \left( {\lambda_{1} - S_{11}} \right)^{2}}}}}\left( {{double}\mspace{14mu}{signs}\mspace{14mu}{in}\mspace{14mu}{same}\mspace{14mu}{order}} \right)} & \left\lbrack {{Equation}\mspace{20mu} 9} \right\rbrack\end{matrix}$are derived. Regardless of the sign of u₁₁ and u₁₂ being plus or minus,the axis represented by the first principal component is the same.

On the other hand, assuming that one of the two eigenvalues havingsmaller variance (eigenvalue of the second principal component) is λ₂,an eigenvector U_(min) corresponding to the eigenvalue λ₂ is

$\begin{matrix}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix} & \left\lbrack {{Equation}\mspace{20mu} 11} \right\rbrack\end{matrix}$which establishes

$\begin{matrix}{{\begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}} = {\lambda_{2}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}}} & \left\lbrack {{Equation}\mspace{20mu} 10} \right\rbrack\end{matrix}$

where u₂₁ ²+u₂₂ ²=1

By solving u₂₁ and u₂₂,

$\begin{matrix}\begin{matrix}{u_{21} = {\pm \frac{S_{12}}{\sqrt{S_{12}^{2} - \left( {\lambda_{2} - S_{11}} \right)^{2}}}}} \\{u_{22} = {\pm \frac{\lambda_{2} - S_{11}}{\sqrt{S_{12}^{2} - \left( {\lambda_{2} - S_{11}} \right)^{2}}}}} \\\left( {{double}\mspace{14mu}{signs}\mspace{14mu}{in}\mspace{14mu}{same}\mspace{14mu}{order}} \right)\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$are derived. Regardless of the sign of u₂₁ and u₂₂ being plus or minus,the axis represented by the second principal component is the same.

From the foregoing, the eigenvector U_(max) of the first principalcomponent and the eigenvector U_(min) of the second principal componentare derived as follows.

$\begin{matrix}{{\overset{\rightarrow}{U_{\max}} = \begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}},{\overset{\rightarrow}{U_{\min}} = \begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

With respect to the eigenvectors U_(max) and U_(min) obtained from thecovariance matrix, it is not possible to predict in which quadrant itappears due to its nature. If the quadrant in which the eigenvectorU_(max) appears changes per frame, U_(max)'s mutually cancel themselvesupon ensemble-averaging U_(max)'s per block later. If the quadrant inwhich the eigenvector U_(min) appears changes per frame, U_(min)'smutually cancel themselves upon ensemble-averaging U_(min)'s per blocklater. Therefore, a conversion operation is performed for fixing thequadrants in which the eigenvectors U_(max) and U_(min) appear. Forexample, when fixing the eigenvector U_(max) to the first quadrant andthe eigenvector U_(min) to the fourth quadrant, it can be realized bythe following conversion operation.

Of the eigenvectors U_(max) and U_(min),

-   -   one existing in the first quadrant (positive, positive) or the        third quadrant (negative, negative) is given as U_(max)′, and    -   one existing in the second quadrant (negative, positive) or the        fourth quadrant (positive, negative) is given as U_(min)′. Then,        conversion is executed such that    -   when U_(max)′ exists in the first quadrant, U_(max)=U_(max)′    -   when U_(max)′ exists in the third quadrant, U_(max)=−U_(max)′    -   when U_(min)′ exists in the second quadrant, U_(min)=−U_(min)′    -   when U_(min)′ exists in the fourth quadrant, U_(min)=U_(min)′.

Through such a conversion operation, the quadrants in which theeigenvectors U_(max) and U_(min) appear can be fixed.

Onto the eigenvector U_(max) of the first principal component and theeigenvector U_(min) of the second principal component that are obtainedas described above, a column vector of the observation matrix B given as

$\begin{matrix}{\overset{\rightarrow}{b} = \begin{pmatrix}x_{L\; n} \\x_{R\; n}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$is projected. A value of an output signal x_(max) obtained by projectingthe observation matrix B onto the eigenvector U_(max) is derived asx _(max)={right arrow over (b)}●{right arrow over (U_(max))}(●represents inner product)  [Equation 15]

Further, a value of an output signal x_(min) obtained by projecting theobservation matrix B onto the eigenvector U_(min) is derived asx _(min)={right arrow over (b)}●{right arrow over (U_(min))}(●represents inner product)  [Equation 16]

The transfer function calculation process of the transfer functioncalculating means 502 will be described.

(In Case of Fixed Type Operation)

A filter characteristic of the orthogonalizing filter 500 is given as U.The filter characteristic U is a characteristic that projects the inputsignals x_(L) and x_(R) onto the mutually uncorrelated signals x_(max)and x_(min) based on the principal component analysis, and the followingrelationship is established.

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 17} \right\rbrack \\{\left. \begin{matrix}{\begin{pmatrix}\overset{\rightarrow}{x_{\max}} \\\overset{\rightarrow}{x_{\min}}\end{pmatrix} = {U\begin{pmatrix}\overset{\rightarrow}{x_{L}} \\\overset{\rightarrow}{x_{R}}\end{pmatrix}}} \\{U = \begin{pmatrix}u_{11} & u_{12} \\u_{21} & u_{22}\end{pmatrix}}\end{matrix} \right\}\mspace{11mu}}\end{matrix} & (95)\end{matrix}$

An inverse characteristic of the filter characteristic U is given as V.The inverse filter characteristic V is a characteristic that restoresthe signals x_(max) and x_(min) to the original signals x_(L) and x_(R),and the following relationship is established.

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 18} \right\rbrack \\{\left. \begin{matrix}{{UV} = {{VU} = {I\mspace{14mu}\left( {I\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{unit}\mspace{14mu}{matrix}} \right)}}} \\{V = \begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix}}\end{matrix} \right\}\mspace{14mu}}\end{matrix} & (96)\end{matrix}$

Output signals y_(L) and y_(R) of the microphones MC(L) and MC(R) areexpressed asY _(L) =H _(LL) ·X _(L) +H _(RL) ·X _(R)  (97)Y _(R) =H _(LR) ·X _(L) +H _(RR) ·X _(R)  (98).

From the equations (95) and (96),

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 19} \right\rbrack \\{\begin{pmatrix}x_{L} \\x_{R}\end{pmatrix} = {\begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix}\begin{pmatrix}x_{\max} \\x_{\min}\end{pmatrix}}}\end{matrix} & (99)\end{matrix}$hence, assuming that frequency-axis expressions of x_(max) and x_(min)are respectively given as X_(max) and X_(min), the equations (97) and(98) respectively becomeY _(L) =H _(LL)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+H _(RL)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))  (97′)Y _(R) =H _(LR)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+H _(RR)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))  (98′).

When both sides of the equation (97′) are multiplied by complexconjugates X*_(max) and X*_(min) of X_(max) and X_(min) (i.e. derivingcross spectra) and ensemble-averaged, because X_(max) and X_(min) aremutually orthogonal, expected values of X*_(max)·X_(min) andX*_(min)·X_(max) become zero, respectively, so that the following twoequations are obtained (note: E[ ] represents the ensemble average).E[X* _(max) ·Y _(L) ]=E[X* _(max) ·H _(LL) ·v ₁₁ ·X _(max) +X* _(max) ·H_(RL) ·v ₂₁ ·X _(max)]  (100)E[X* _(min) ·Y _(L) ]=E[X* _(min) ·H _(LL) ·v ₁₂ ·X _(min) +X* _(min) ·H_(RL) ·v ₂₂ ·X _(min)]  (101)

Similarly, when both sides of the equation (98′) are multiplied bycomplex conjugates X*_(max) and X*_(min) of X_(max) and X_(min) andensemble-averaged, the following two equations are obtained.E[X* _(max) ·Y _(R) ]=E[X* _(max) ·H _(LR) ·v ₁₁ ·X _(max) +X* _(max) ·H_(RR) ·v ₂₁ ·X _(max)]  (102)E[X* _(min) ·Y _(R) ]=E[X* _(min) ·H _(LR) ·v ₁₂ ·X _(min) +X* _(min) +H_(RR) ·v ₂₂ ·X _(min)]  (103)

Here, if a change of the eigenvalues λ is small in a time period ofperforming ensemble averaging,

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 20} \right\rbrack \\\left. \begin{matrix}{U = {{\begin{pmatrix}u_{11} & u_{12} \\u_{21} & u_{22}\end{pmatrix} \approx {E\lbrack U\rbrack}} = \begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}} \\{V = {{\begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix} \approx {E\lbrack V\rbrack}} = \begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} \right\}\end{matrix} & (104)\end{matrix}$is established, so that the equations (100) to (103) are rewritten asthe following equations (100′) to (103′).E[X* _(max) ·Y _(L) ]≈E[|X _(max)|² ]·H _(LL) ·E[v ₁₁ ]+E[|X _(max)|²]·H _(RL) ·E[v ₂₁]  (100′)E[X* _(min) ·Y _(L) ]≈E[|X _(min)|² ]·H _(LL) ·E[v ₁₂ ]+E[|X _(min)|²]·H _(RL) ·E[v ₂₂]  (101′)E[X* _(max) ·Y _(R) ]≈E[|X _(max)|² ]·H _(LR) ·E[v ₁₁ ]+E[|X _(max)|²]·H _(RR) ·E[v ₂₁]  (102′)E[X* _(min) ·Y _(R) ]≈E[|X _(min)|²]·H_(LR) ·E[v ₁₂ ]+E[|X_(min)|²]·H_(RR) ·E[v ₂₂]  (103′)

When both sides of the equations (100′) and (102′) are divided byE[|X_(max)|²] and both sides of the equations (101′) and (103′) aredivided by E[|X_(min)|²], respectively,E[X* _(max) ·Y _(L) ]/E[|X _(max)|² ]≈H _(LL) ·E[v ₁₁ ]+H _(RL) ·E[v ₂₁]E[X* _(min) ·Y _(L) ]/E[|X _(min)|² ]≈H _(LL) ·E[v ₁₂ ]+H _(RL) ·E[v ₂₂]E[X* _(max) ·Y _(R) ]/E[|X _(max)|² ]≈H _(LR) ·E[v ₁₁ ]+H _(RR) ·E[v ₂₁]E[X* _(min) ·Y _(R) ]/E[|X _(min)|² ]≈H _(LR) ·E[v ₁₂ ]+H _(RR) ·E[v ₂₂]hence

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 21} \right\rbrack \\{\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix} \approx {\begin{pmatrix}H_{LL} & H_{RL} \\H_{LR} & H_{RR}\end{pmatrix}\begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (105)\end{matrix}$is obtained. From E[U]·E[V]≈I, the equation (105) is rewritten as

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 22} \right\rbrack \\{\begin{pmatrix}H_{LL} & H_{RL} \\H_{LR} & H_{RR}\end{pmatrix} \approx {\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix}\begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (106)\end{matrix}$

Impulse responses h_(LL), h_(RL), h_(LR) and h_(RR) obtained by applyingthe inverse Fourier transformation to the transfer functions H_(LL),H_(RL), H_(LR) and H_(RR) derived from the equation (106) are the filtercharacteristics to be set to the filter means 40-1, 40-2, 40-3 and 40-4,respectively. Therefore, the transfer function calculating means 502derives the respective transfer functions H_(LL), H_(RL), H_(LR) andH_(RR) based on the signals x_(max) and x_(min) outputted from theorthogonalizing filter 500, the filter characteristic U of theorthogonalizing filter 500 and the output signals y_(L) and y_(R) of themicrophones MC(L) and MC(R), derives the impulse responses h_(LL),h_(RL), h_(LR) and h_(RR) by applying the inverse Fourier transformationto those derived transfer functions, sets the derived impulse responsesto the filter means 40-1, 40-2, 40-3 and 40-4, respectively, andfurther, updates the impulse responses by repeating this calculation persuitably determined prescribed time period (e.g. time period ofperforming ensemble averaging).

(In Case of Adaptive Type Operation)

Assuming that the filter characteristics set to the filter means 40-1,40-2, 40-3 and 40-4 are given as H^_(LL), H^_(RL), H^_(LR) and H^_(RR)(h^_(LL), h^_(RL), h^_(LR) and h^_(RR) when expressed in terms of theimpulse responses), the signals e_(L) and e_(R) outputted from thesubtracters 48 and 50 of FIG. 28 are expressed asE _(L)=(H _(LL) ·X _(L) +H _(RL) ·X _(R))−(H^ _(LL) ·X _(L) +H^ _(RL) ·X_(R))  (107)E _(R)=(H _(LR) ·X _(L) +H _(RR) ·X _(R))−(H^ _(LR) ·X _(L) +H^ _(RR) ·X_(R))  (108).

From the foregoing equation (99), the equations (107) and (108)respectively becomeE _(L)=(H _(LL) −H^_(LL))(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+(H _(RL) −H^_(RL))(v ₂₁ ·X _(max) +v ₂₂ ·X _(min))  (107′)E _(R)=(H _(LR) −H^_(LR))(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+(H _(RR) −H^_(RR))(v ₂₁ ·X _(max) +V ₂₂ ·X _(min))  (108′).

When the estimated errors of the transfer functions are given asΔH _(LL) =H _(LL) −H^_(LL)ΔH _(RL) =H _(RL) −H^_(RL)ΔH _(LR) =H _(LR) −H^_(LR)ΔH _(RR) =H _(RR) −H^_(RR)the equations (107′) and (108′) respectively becomeE _(L) =ΔH _(LL)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+ΔH _(RL)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))  (107″)E _(R) =ΔH _(LR)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+ΔH _(RR)(v ₂₁·X_(max)+v ₂₂ ·X _(min))  (108″).

When both sides of the equation (107″) are multiplied by complexconjugates X*_(max) and X*_(min) of X_(max) and X_(min) (i.e. derivingcross spectra) and ensemble-averaged, because X_(max) and X_(min) aremutually orthogonal, expected values of X*_(max)·X_(min) andX*_(min)·X_(max) become zero, respectively, so that the following twoequations are obtained (note: E[ ] represents the ensemble average).E[X* _(max) ·E _(L) ]=E[X* _(max) ·ΔH _(LL) ·v ₁₁ ·X _(max) +X* _(max)·ΔH _(RL) ·v ₂₁ ·X _(max)]  (109)E[X* _(min) ·E _(L) ]=E[X* _(min) ·ΔH _(LL) ·v ₁₂ ·X _(min) +X* _(min)·ΔH _(RL) ·v ₂₂ ·X _(min)]  (110)

Similarly, when both sides of the equation (108″) are multiplied bycomplex conjugates X*_(max) and X*_(min) of X_(max) and X_(min) andensemble-averaged, the following two equations are obtained.E[X* _(max) ·E _(R) ]=E[X* _(max) ·ΔH _(LR) ·v ₁₁ ·X _(max) +X* _(max)·ΔH _(RR) ·v ₂₁ ·X _(max) ]  (111)E[X* _(min) ·E _(R) ]=E[X* _(min) ·ΔH _(LR) ·v ₁₂ ·X _(min) +X* _(min)·ΔH _(RR) ·v ₂₂ ·X _(min)]  (112)

Here, if a change of the eigenvalues λ is small in a time period ofperforming ensemble averaging, the foregoing equation (104) isestablished so that the equations (109) to (112) are rewritten as thefollowing equations (109′) to (112′).E[X* _(max) ·E _(L) ]≈E[|X _(max)|² ]·ΔH _(LL) ·E[v ₁₁ ]+E[|X _(max)|²]·ΔH _(RL) ·E[v ₂₁]  (109′)E[X* _(min) ·E _(L) ]≈E[|X _(min)|² ]·ΔH _(LL) ·E[v ₁₂ ]+E[|X _(min)|²]·ΔH _(RL) ·E[v ₂₂]  (110′)E[X* _(max) ·E _(R) ]≈E[|X _(max)|² ]·ΔH _(LR) ·E[v ₁₁ ]+E[|X _(max)|²]·ΔH _(RR) ·E[v ₂₁]  (111′)E[X* _(min) ·E _(R) ]≈E[|X _(min)|² ]·ΔH _(LR) ·E[v ₁₂ ]+E[|X _(min)|²]·ΔH _(RR) ·E[v ₂₂]  (112′)

When both sides of the equations (109′) and (111′) are divided byE[|X_(max)|²] and both sides of the equations (110′) and (112′) aredivided by E[|X_(min)|²], respectively,E[X* _(max) ·E _(L) ]/E[|X _(max)|² ]≈ΔH _(LL) ·E[v ₁₁ ]+ΔH _(RL) ·E[v₂₁]E[X* _(min) ·E _(L) ]/E[|X _(min)|² ]≈ΔH _(LL) ·E[v ₁₂ ]+ΔH _(RL) ·E[v₂₂]E[X* _(max) ·E _(R) ]/E[|X _(max)|² ]≈ΔH _(LR) ·E[v ₁₁ ]+ΔH _(RR) ·E[v₂₁]E[X* _(min) ·E _(R) ]/E[|X _(min)|² ]≈ΔH _(LR) ·E[v ₁₁ ]+ΔH _(RR) ·E[v₂₂]hence

$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 23} \right\rbrack \\{\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix} \approx {\begin{pmatrix}{\Delta\; H_{LL}} & {\Delta\; H_{RL}} \\{\Delta\; H_{LR}} & {\Delta\; H_{RR}}\end{pmatrix}\begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (113)\end{matrix}$is obtained. From E[U]·E[V]≈I, the equation (113) is rewritten as

$\begin{matrix}{\left\lbrack {{Equation}\mspace{14mu} 24} \right\rbrack{\begin{pmatrix}{\Delta\; H_{LL}} & {\Delta\; H_{RL}} \\{\Delta\; H_{LR}} & {\Delta\; H_{RR}}\end{pmatrix} \approx {\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix}\begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}}} & (114)\end{matrix}$

Using the estimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR) derivedfrom the equation (114), the filter characteristics of the filter means40-1, 40-2, 40-3 and 40-4 are updated per suitably determined prescribedtime period (e.g. time period of performing ensemble averaging). Forexample, assuming that impulse responses h_(LL), h_(RL), h_(LR) andh_(RR) after K-th updating are given as h_(LL)(k), h_(RL)(k), h_(LR)(k)and h_(RR)(k), using impulse responses Δh_(LL), ΔH_(RL), Δh_(LR) andΔh_(RR) corresponding to the derived estimated errors ΔH_(LL), ΔH_(RL),ΔH_(LR) and ΔH_(RR),h _(LL)(k+1)=h _(LL)(k)+αΔh _(LL)h _(RL)(k+1)=h _(RL)(k)+αΔh _(RL)h _(LR)(k+1)=h _(LR)(k)+αΔh _(LR)h _(RR)(k+1)=h _(RR)(k)+αΔh _(RR).

Using these updating equations, (k+1)th impulse responses h_(LL)(k+1),h_(RL)(k+1), h_(LR)(k+1) and h_(RR)(k+1) are derived and set to thefilter means 40-1, 40-2, 40-3 and 40-4, respectively, which is repeatedper suitably determined prescribed time period (e.g. time period ofperforming ensemble averaging).

FIG. 30 shows functional blocks of the orthogonalizing filter 500 ofFIG. 28. The input stereo signals x_(L) and x_(R) are inputted frominput ends 506 and 508, respectively. Covariance matrix calculatingmeans 510 derives a covariance matrix S of the input stereo signalsx_(L) and x_(R) per frame. Based on the derived covariance matrix S,eigenvector calculating means 512 derives coefficients u₁₁, u₁₂, u₂₁ andu₂₂ of eigenvectors U_(max) and U_(min) of the first and secondprincipal components per frame. The derived coefficients u₁₁, u₂₁, u₁₂and u₂₂ are set to coefficient multipliers 514, 516, 518 and 520,respectively. The coefficient multipliers 514 and 516 give thecoefficients u₁₁ and u₂₁ to the input signal x_(L) to derive x_(L)·u₁₁and x_(L)·u₂₁, respectively. The coefficient multipliers 518 and 520give the coefficients u₁₂ and u₂₂ to the input signal x_(R) to derivex_(R)·u₁₂ and x_(R)·u₂₂, respectively. An adder 522 derivesx_(L)·u₁₁+x_(R)·u₁₂ as a signal x_(max) obtained by projecting the inputstereo signals x_(L) and x_(R) onto the eigenvector U_(max). An adder524 derives x_(L)·u₂₁+x_(R)·u₂₂ as a signal x_(min) obtained byprojecting the input stereo signals x_(L) and x_(R) onto the eigenvectorU_(min). The signals x_(max) and x_(min) are respectively outputted fromoutput ends 526 and 528, while the coefficients u₁₁, u₂₁, u₁₂ and u₂₂ ofthe eigenvectors are outputted from an output end 530.

FIG. 31 shows functional blocks of the transfer function calculatingmeans 502 of FIG. 28. The signals x_(max) and x_(min) are respectivelyinputted from input ends 532 and 534, while the coefficients u₁₁, u₂₁,u₁₂ and u₂₂ of the eigenvectors are inputted from an input end 536. FFTmeans 538 applies the fast Fourier transformation to the signal x_(max).Complex conjugate calculating means 540 calculates a complex conjugateX*_(max) of X_(max). Power spectrum calculating means 542 calculatesX_(max)·X*_(max)=|X_(max)|². Ensemble averaging means 544 calculatesE[|X_(max)|²]. FFT means 546 applies the fast Fourier transformation tothe signal x_(min). Complex conjugate calculating means 548 calculates acomplex conjugate X*_(min) of X_(min). Power spectrum calculating means550 calculates X_(min)·X*_(min)=|X_(min)|². Ensemble averaging means 552calculates E[|X_(min)|²].

An input end 554 is inputted with an output signal y_(L) of themicrophone MC(L) (or output signal E_(L) of the subtracter 48). FFTmeans 558 applies the fast Fourier transformation to the signal y_(L)(or signal E_(L)). Cross spectrum calculating means 560 derivesX*_(max)·Y_(L) (or X*_(max)·E_(L)), and ensemble averaging means 562derives E[X*_(max)·Y_(L)] (or E[X*_(max)·E_(L)]). Cross spectrumcalculating means 564 derives X*_(min)·Y_(L) (or X*_(min)·E_(L)), andensemble averaging means 566 derives E[X*_(min)·Y_(L)] (orE[X*_(min)·E_(L)]).

An input end 568 is inputted with an output signal y_(R) of themicrophone MC(R) (or output signal E_(R) of the subtracter 50). FFTmeans 570 applies the fast Fourier transformation to the signal y_(R)(or signal E_(R)). Cross spectrum calculating means 572 derivesX*_(max)·Y_(R) (or X*_(max)·E_(R)), and ensemble averaging means 574derives E[X*_(max)·Y_(R)] (or E[X*_(max)·E_(R)]). Cross spectrumcalculating means 576 derives X*_(min)·Y_(R) (or X*_(min)·E_(R)), andensemble averaging means 578 derives E[X*_(min)·Y_(R)] (orE[X*_(min)·E_(R)]).

Composite transfer function calculating means 580 derives the first termon the right side of the equation (106) {or the equation (114)} based onE[|X_(max)|²], E[|X_(min)|²], E[X*_(max)·Y_(L)] (or E[X*_(max)·E_(L)]),E[X*_(min)·Y_(L)] (or E[X*_(min)·E_(L)]), E[X*_(max)·Y_(R)] (orE[X*_(max)·E_(R)]), and E[X*_(min)·Y_(R)] (or E[X*_(min)·E_(R)]) whichare derived as described above. Averaging means 582 averages thecoefficients u₁₁, u₂₁, u₁₂ and u₂₂ of the eigenvectors individually toderive E[v₁₁], E[v₁₂], E[v₂₁] and E[v₂₂].

Based on the outputs of the composite transfer function calculatingmeans 580 and the averaging means 582, transfer function calculatingmeans 584 performs a calculation of the right side of the equation (106){or the equation (114)} to derive individual transfer functions H_(LL),H_(RL), H_(LR) and H_(RR) (or their estimated errors ΔH_(LL), ΔH_(RL),ΔH_(LR) and ΔH_(RR)). Inverse FFT means 586 applies the inverse fastFourier transformation to the derived transfer functions H_(LL), H_(RL),H_(LR) and H_(RR) (or their estimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR)and ΔH_(RR)) to derive corresponding impulse responses h_(LL), h_(RL),h_(LR) and h_(RR) (or their estimated errors Δh_(LL), Δh_(RL), Δh_(LR)and Δh_(RR)), and outputs them from output ends 588, 590, 592 and 594,respectively. As shown in FIG. 32, the transfer function calculatingmeans 584 and the inverse FFT means 586 may be exchanged therebetween intheir positions.

With respect to the thus structured stereo echo canceller 16, 24 of FIG.28, the results of carrying out adaptive type operation simulations areshown in FIGS. 33 to 48 in connection with one audio transfer system.FIGS. 33 to 40 show time-domain variations in echo cancellation amount,while FIGS. 41 to 48 show time-domain variations in transfer functionestimated error. Herein, the simulations were conducted under thefollowing conditions.

-   Sampling Frequency: 11.025 kHz-   The Number of Samples in One Frame: 4096 samples-   The Number of Frames in One Block: variable (2 frames, 4 frames, 8    frames, 16 frames)-   Update Period of Filter Characteristic: per block (about 0.75    seconds in case of the number of frames in one block being two,    about 1.5 seconds in case of 4 frames, about 3 seconds in case of 8    frames, about 6 seconds in case of 16 frames)-   The Mean Number of Times of Ensemble Averaging in One Block: 31    times in case of the number of frames in one block being two, 63    times in case of 4 frames, 127 times in case of 8 frames, 255 times    in case of 16 frames {For the purpose of increasing the mean number    of times, as shown in FIG. 49, one frame is divided into 16    intervals, and data corresponding to one frame is extracted from the    head of each divisional interval while overlapping data extraction    successively, thereby to derive individual parameter values to be    ensemble-averaged, and the derived individual parameter values are    respectively ensemble-averaged in one block. Therefore, the mean    number of times N of ensemble averaging becomes such that N=(16×the    number of frames in one block−1).}

In any case, the filter characteristics are not set in the first block,and the initial setting is implemented in the second block, andthereafter, updating is performed per block. The axis of ordinates (dB)is defined such that 0 dB represents the initial state where the filtercharacteristics are not set. Differences in simulation condition withrespect to FIGS. 33 to 48 are as follows.

TABLE 2 Presence/Absence Number of Frames Figure Number of Double Talkin One Block FIG. 33, FIG. 41 Absent 2 FIG. 34, FIG. 42 4 FIG. 35, FIG.43 8 FIG. 36, FIG. 44 16 FIG. 37, FIG. 45 Present 2 FIG. 38, FIG. 46 4FIG. 39, FIG. 47 8 FIG. 40, FIG. 48 16

The simulation results of FIGS. 33 to 48 are considered.

(1) In Case of Absence of Double Talk

Since the rising speed of the echo cancellation amount is FIG. 33>FIG.34>FIG. 35>FIG. 36, the rising speed of the echo cancellation amountbecomes faster as one block (update period) becomes shorter (the numberof frames per block becomes smaller). When the number of frames in oneblock is 2, 4 or 8, the echo cancellation amount of about 25 dB isobtained (FIG. 33, 34 or 35). When the number of frames in one blockincreases, i.e. 16 frames, the echo cancellation amount requires a longtime for reduction thereof (FIG. 36). Since the estimated errorconvergence speed is FIG. 41>FIG. 42>FIG. 43>FIG. 44, the estimatederror convergence speed becomes faster as one block becomes shorter.

(2) In Case of Presence of Double Talk

When the number of frames in one block is 2 or 4 frames, the echocancellation amount is not increased (FIG. 37 or 38), and the estimatederror is not converged (FIG. 45 or 46) and thus can not be estimated.When the number of frames in one block is 8, the echo cancellationamount of about 15 dB is obtained (FIG. 39), and the estimated error isconverged to about −6 dB (FIG. 47). When the number of frames in oneblock is 16, the echo cancellation amount of about 17 dB is obtained(FIG. 40), and the estimated error is converged to about 10 dB and thusthe fairly stable estimation can be achieved.

From the foregoing simulation results, the followings can be said.

-   (a) When the double talk is not detected, the convergence of the    estimated error can be quickened by relatively shortening the update    period of the filter characteristic.-   (b) When the double talk is detected, the estimated error can be    fully converged by relatively prolonging the update period of the    filter characteristic.

Therefore, as described above, the transfer function calculating means502 of FIG. 28 makes relatively longer the update period of the filtercharacteristics of the filter means 40-1 to 40-4 while the double talkis detected, whereas makes relatively shorter the update period of thefilter characteristics while the double talk is not detected. This makesit possible to fully converge the estimated errors when the double talkexists, and further, quicken the convergence of the estimated errorswhen there is no double talk.

FIG. 50 shows a modification of the stereo echo canceller 16, 24 of FIG.28. The same symbols are used with respect to those portions common toFIG. 28. In this modification, an orthogonalizing filter is disposed onsignal lines of loudspeakers SP(L) and SP(R). An inverse filter 596 hasan inverse characteristic of the orthogonalizing filter 500 {the inversefilter characteristic V of the foregoing equation (96)}, thereby torestore output signals x_(max) and x_(min) of the orthogonalizing filter500 to the original signal x_(L) and x_(R), and feeds them to theloudspeakers SP(L) and SP(R).

In the foregoing embodiments, the number of the loudspeakers is two andthe number of the microphones is two. However, it may also be configuredthat the number of the loudspeakers is two, while the number of themicrophones is one. FIG. 51 shows a structural example as a result ofmodifying FIG. 1 in such a manner. The same symbols are used withrespect to those portions common to FIG. 1. Left/right two-channelstereo signals x_(L) and x_(R) transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areoutputted from sound output ends SO(L) and SO(R) as they are (i.e. notthrough sum/difference signal producing means 52), and reproduced atloudspeakers SP(L) and SP(R), respectively.

Filter means 40-1 is set with an impulse response corresponding to atransfer function H_(L) between the loudspeaker SP(L) and a microphoneMC and performs, using such an impulse response, a convolutioncalculation of a signal x_(L) to be outputted from the sound output endSO(L), thereby producing an echo cancel signal EC1 corresponding to asignal y_(L) obtained such that the signal x_(L) outputted from thesound output end SO(L) is reproduced at the loudspeaker SP(L), collectedby the microphone MC and inputted into a sound input end SI. Filtermeans 40-3 is set with an impulse response corresponding to a transferfunction H_(R) between the loudspeaker SP(R) and the microphone MC andperforms, using such an impulse response, a convolution calculation of asignal x_(R) to be outputted from the sound output end SO(R), therebyproducing an echo cancel signal EC3 corresponding to a signal y_(R)obtained such that the signal x_(R) outputted from the sound output endSO(R) is reproduced at the loudspeaker SP(R), collected by themicrophone MC and inputted into the sound input end SI. An adder 44performs a calculation of EC1+EC3. A subtracter 48 subtracts an echocancel signal EC1+EC3 from a collected audio signal y(=y_(L)+y_(R)) ofthe microphone MC inputted from the sound input end SI, thereby toperform echo cancellation. An echo-canceled signal e(=e_(L)+e_(R)) isoutputted from a line output end LO and transmitted toward the spot onthe counterpart side.

The sum/difference signal producing means 52 performs addition, using anadder 54, of the left/right two-channel stereo signals x_(L) and x_(R)inputted into the line input ends LI(L) and LI(R) so as to produce a sumsignal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Transfer function calculating means 58implements a cross-spectrum calculation between the sum signal x_(M) andthe difference signal x_(S) produced by the sum/difference signalproducing means 52 and the signal e outputted from the subtracter 48and, based on this cross-spectrum calculation, sets filtercharacteristics (impulse responses) of the filter means 40-1 and 40-3.Specifically, upon starting the system, the filter characteristics ofthe filter means 40-1 and 40-3 are not set, i.e. coefficients are allset to zero, so that the echo cancel signals EC1 and EC3 are zero, andthus the collected audio signal of the microphone MC itself is outputtedfrom the subtracter 48. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(M) and the difference signal x_(S) produced by thesum/difference signal producing means 52 and the collected audio signale of the microphones MC outputted from the subtracter 48 and, based onthis cross-spectrum calculation, derives transfer functions of two audiotransfer systems between the loudspeakers SP(L) and SP(R) and themicrophone MC, respectively, and implements initial setting of thefilter characteristics of the filter means 40-1 and 40-3 to valuescorresponding to such transfer functions. After the initial setting,since the echo cancel signals are produced by the filter means 40-1 and40-3, the echo cancel error signal e corresponding to a differencesignal between the collected audio signal of the microphone MC and theecho cancel signal EC1+EC3 is outputted from the subtracter 48.Therefore, at this time, the transfer function calculating means 58performs the cross-spectrum calculation between the sum signal x_(M) andthe difference signal x_(S) produced by the sum/difference signalproducing means 52 and the echo cancel error signal e outputted from thesubtracter 48 and, based on this cross-spectrum calculation, derivesestimated errors of the transfer functions of the two audio transfersystems between the loudspeakers SP(L) and SP(R) and the microphone MC,respectively, and updates the filter characteristics of the filter means40-1 and 40-3 to values that cancel the estimated errors, respectively.By repeating this updating operation per prescribed time period, theecho cancel error can be converged to a minimum value. Further, even ifthe transfer functions change due to movement of the microphonepositions or the like, the echo cancel error can be converged to aminimum value by sequentially updating the filter characteristics of thefilter means 40-1 and 40-3 depending thereon.

Correlation detecting means 60 detects a correlation between the sumsignal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted. Also in the embodiments other than FIG. 1, it can beconfigured that the number of the loudspeakers is two, while the numberof the microphones is one.

The stereo echo canceller 16, 24 shown in each of the foregoingembodiments can be formed by the dedicated hardware or can also berealized through software processing in a general computer. For example,the functions of the respective blocks as shown in FIG. 1 and so forthcan be accomplished by a CPU (Central Processing Unit) and storing meanssuch as a RAM or ROM constituting the computer. Namely, the CPU may becaused to function as the echo canceller according to a program storedin the storing means such as the ROM or RAM.

In the foregoing embodiments, the description has been made about thecase where the two-channel stereo signals are handled. However, the echocancellation can also be implemented using the technique of thisinvention with respect to those signals of three channels or more havinga correlation with each other.

1. A stereo echo canceller associated to a space provided therein with two loudspeakers and two microphones for forming four audio transfer systems through which stereo sounds are reproduced by said respective loudspeakers and are collected by said respective microphones, the canceller comprising: first and second filter sections that are provided corresponding to the first and second microphones for subjecting an audio signal supplied to the first loudspeaker to convolution calculations so as to produce first and second echo cancel signals, respectively; third and fourth filter sections that are provided corresponding to the first and second microphones for subjecting another audio signal supplied to the second loudspeaker to convolution calculations so as to produce third and fourth echo cancel signals, respectively; a first subtracting section that performs echo cancellation by subtracting said first and third echo cancel signals from a collected audio signal of the first microphone; and a second subtracting section that performs echo cancellation by subtracting said second and fourth echo cancel signals from another collected audio signal of the second microphone, wherein said stereo echo canceller further comprises a transfer function calculating section that respectively derives filter characteristics corresponding to transfer functions of said four audio transfer systems based on a cross-spectrum calculation between a sum signal and difference signal of stereo audio signals to be reproduced by said respective loudspeakers and the collected audio signals of said respective microphones, thereby to set said derived filter characteristics to corresponding ones of said first to fourth filter sections, respectively.
 2. A stereo echo canceller as recited in claim 1, further comprising: an input section that inputs said stereo audio signals; a sum/difference signal producing section that produces said sum signal and said difference signal of the stereo audio signals inputted from said input section; and a main signal transmission system that transmits the stereo audio signals inputted from said input section to said respective loudspeakers without passing through said sum/difference signal producing section, wherein said transfer function calculating section derives the filter characteristics corresponding to the transfer functions of said four audio transfer systems based on the cross-spectrum calculation between the sum signal and difference signal produced by said sum/difference signal producing section and the collected audio signals of said respective microphones, and sets the derived filter characteristics to corresponding ones of said first to fourth filter sections, respectively.
 3. A stereo sound transfer apparatus associated to two spaces each forming said four audio transfer systems, wherein the stereo echo canceller recited in claim 1 is arranged in each space, so that the stereo audio signals, which have been echo-canceled by said stereo echo cancellers, are transmitted between said two spaces.
 4. A stereo echo canceller associated to a space provided therein with two loudspeakers and two microphones for forming four audio transfer systems through which stereo sounds are reproduced by said respective loudspeakers and are collected by said respective microphones, the canceller comprising: first and second filter sections that are provided corresponding to the first and second microphones for subjecting an audio signal supplied to the first loudspeaker to convolution calculations so as to produce first and second echo cancel signals, respectively; third and fourth filter sections that are provided corresponding to the first and second microphones for subjecting another audio signal supplied to the second loudspeaker to convolution calculations so as to produce third and fourth echo cancel signals, respectively; a first subtracting section performs echo cancellation by subtracting said first and third echo cancel signals from a collected audio signal of the first microphone; and a second subtracting section that performs echo cancellation by subtracting said second and fourth echo cancel signals from another collected audio signal of the second microphone, wherein said stereo echo canceller further comprises a transfer function calculating section respectively derives estimated errors of transfer functions of said four audio transfer systems based on a cross-spectrum calculation between respective one of a sum signal and a difference signal of stereo audio signals to be reproduced by said respective loudspeakers and respective one of echo cancel error signals obtained by subtracting the corresponding echo cancel signals from the collected audio signals of said two microphones, thereby to update filter characteristics of said first to fourth filter sections to values that cancel said estimated errors, respectively.
 5. A stereo echo canceller as recited in claim 4, further comprising: an input section that inputs said stereo audio signals; a sum/difference signal producing section that produces said sum signal and said difference signal of the stereo audio signals inputted from said input section; and a main signal transmission system that transmits the stereo audio signals inputted from said input section to said respective loudspeakers without passing through said sum/difference signal producing section, wherein said transfer function calculating section derives the estimated errors of the transfer functions of said four audio transfer systems based on the cross-spectrum calculation between the sum signal and difference signal produced by said sum/difference signal producing section and the respective echo cancel error signals, and updates the filter characteristics of said first to fourth filter sections to the values that cancel said estimated errors, respectively.
 6. A stereo echo canceller as recited in claim 4, further comprising a correlation detecting section that detects a correlation between the sum signal and the difference signal of said stereo audio signals, and that stops the updating of said filter characteristics when a value of said correlation is no less than a predetermined value.
 7. A multi-channel echo cancel method associated to a space provided therein with a plurality of loudspeakers and at least one microphone for forming a plurality of audio transfer systems, the method comprising: inputting multi-channel audio signals from an outside, which have a correlation with each other, and which are reproduced by said respective loudspeakers and collected by said at least one microphone through the audio transfer systems; estimating individual transfer functions of said plurality of said audio transfer systems so as to set corresponding filter characteristics, respectively; producing echo cancel signals respectively by applying said set filter characteristics to corresponding ones of said multi-channel audio signals to be reproduced by said respective loudspeakers; and subtracting said echo cancel signals from corresponding individual collected audio signals of said at least one microphone, thereby performing echo cancellation, wherein, reference signals are determined as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that among said multi-channel audio signals, for deriving said individual transfer functions of the respective audio transfer systems thereby setting said corresponding filter characteristics.
 8. A multi-channel echo cancel method as recited in claim 7, wherein calculation is conducted for respectively deriving the individual transfer functions of the respective audio transfer systems with using the set of the plurality of the low-correlation composite signals as the reference signals, such that the calculation is based on a cross-spectrum calculation between the plurality of the low-correlation composite signals and the individual collected audio signals of the at least one microphone.
 9. A multi-channel sound transfer method associated to two spaces each forming said plurality of said audio transfer systems, wherein the multi-channel echo cancel method recited in claim 7 is carried out respectively in the two spaces, so that the multi-channel audio signals, which have been echo-canceled by performing said method, are transmitted between said two spaces.
 10. A multi-channel echo channel method as recited in claim 7, wherein the multi-channel audio signals being inputted from an outside and having a correlation with each other are reproduced by said respective loudspeakers without lowering the correlation of the inputted multi-channel audio signals.
 11. A multi-channel echo cancel method as recited in claim 7, wherein the multi-channel audio signals being inputted from an outside and having a correlation with each other are provisionally modulated to lower the correlation, then demodulated to restore the correlation, and thereafter reproduced by said respective loudspeakers.
 12. A multi-channel echo cancel method as recited in claim 11, wherein the multi-channel audio signals are provisionally modulated to lower the correlation by one of first or second operations, the first operation adding and subtracting the multi-channel audio signals with each other, the second operation orthogonalizing the multi-channel audio signals with each other.
 13. A multi-channel echo cancel method associated to a space provided therein with a plurality of loudspeakers at least one microphone for forming a plurality of audio transfer systems through which multi-channel audio signals having a correlation with each other are reproduced by said respective loudspeakers and are collected by said at least one microphone, the method comprising: estimating individual transfer functions of said plurality of said audio transfer systems so as to set corresponding filter characteristics, respectively; producing echo cancel signals respectively by applying said set filter characteristics to corresponding ones of said multi-channel audio signals to be reproduced by said respective loudspeakers; and subtracting said echo cancel signals from corresponding individual collected audio signals of said at least one microphone, thereby performing echo cancellation, wherein, reference signals are determined as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that among said multi-channel audio signals, for deriving said individual transfer functions of the respective audio transfer systems, thereby setting said corresponding filter characteristics, wherein calculation is conducted for respectively deriving the individual transfer functions of the respective audio transfer systems with using the set of the plurality of the low-correlation composite signals as the reference signals, such that the calculation is based on a cross-spectrum calculation between the plurality of the low-correlation composite signals and the individual collected audio signals of the at least one microphone, and wherein the calculation of respectively deriving the individual transfer functions of said plurality of the audio transfer systems is performed by combining said multi-channel audio signals through arithmetic operation to produce a plurality of low-correlation composite signals having a lower correlation with each other than that among said multi-channel audio signals, deriving cross spectra by the cross-spectrum calculation between said plurality of the low-correlation composite signals and the individual collected audio signals of the at least one microphone, and ensemble-averaging each of the cross spectra in a predetermined time period for deriving the individual transfer functions of said plurality of the audio transfer systems.
 14. A multi-channel echo cancel method associated to a space provided therein with a plurality of loudspeakers at least one microphone for forming a plurality of audio transfer systems, the method comprising: inputting multi-channel audio signals from an outside, which have a correlation with each other, and which are reproduced by said respective loudspeakers and collected by said at least one microphone through the audio transfer systems; estimating individual transfer functions of said plurality of said audio transfer systems so as to set corresponding filter characteristics, respectively; producing echo cancel signals respectively by applying said set filter characteristics to corresponding ones of said multi-channel audio signals to be reproduced by said respective loudspeakers or a plurality of composite signals obtained by suitably combining said multi-channel audio signals; and subtracting said echo cancel signals from corresponding individual collected audio signals of said at least one microphones, thereby performing echo cancellation, wherein, reference signals are determined as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that among said multi-channel audio signals, for deriving estimated errors of said individual transfer functions of the respective audio transfer systems or said plurality of said composite transfer functions obtained by suitably combining said individual transfer functions, thereby updating said corresponding filter characteristics to values that cancel the estimated errors.
 15. A multi-channel echo cancel method as recited in claim 14, wherein calculation is conducted for respectively deriving the estimated errors of the individual transfer functions of the respective audio transfer systems with using the set of the plurality of the low-correlation composite signals as the reference signals, such that the calculation is based on a cross-spectrum calculation between the plurality of the low-correlation composite signals and echo cancel error signals obtained by subtracting the echo cancel signals from the corresponding individual collected audio signals of said at least one microphone.
 16. A multi-channel echo cancel method associated to a space provided therein with a plurality of loudspeakers and at least one microphone for forming a plurality of audio transfer systems through which multi-channel audio signals having a correlation with each other are reproduced by said respective loudspeakers and are collected by said at least one microphone, the method comprising: estimating individual transfer functions of said plurality of said audio transfer systems so as to set corresponding filter characteristics, respectively; producing echo cancel signals respectively by applying said set filter characteristics to corresponding ones of said multi-channel audio signals to be reproduced by said respective loudspeakers; and subtracting said echo cancel signals from corresponding individual collected audio signals of said at least one microphone, thereby performing echo cancellation, wherein, reference signals are determined as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that among said multi-channel audio signals, for deriving estimated errors of said individual transfer functions of the respective audio transfer systems thereby updating said corresponding filter characteristics to values that cancel the estimated errors, wherein calculation is conducted for respectively deriving the estimated errors of the individual transfer functions of the respective audio transfer systems with using the set of the plurality of the low-correlation composite signals as the reference signals, such that the calculation is based on a cross-spectrum calculation between the plurality of the low-correlation composite signals and echo cancel error signals obtained by subtracting the echo cancel signals from the corresponding individual collected audio signals of said at least one microphone, and wherein the calculation of respectively deriving the estimated errors of the individual transfer functions of said plurality of the audio transfer systems is performed by combining said multi-channel audio signals through an arithmetic operation to produce a plurality of low-correlation composite signals having a lower correlation with each other than that among said multi-channel audio signals, deriving cross spectra by the cross-spectrum calculation between said plurality of the low-correlation composite signals and the echo cancel error signals obtained by subtracting the echo cancel signals from the corresponding individual collected audio signals of said at least one microphone, and ensemble-averaging each of the cross spectra in a predetermined time period for deriving the estimated errors of the individual transfer functions of said plurality of the audio transfer systems.
 17. A multi-channel echo cancel method associated to a space provided therein with a plurality of loudspeakers and at least one microphone for forming a plurality of audio transfer systems through which multi-channel audio signals having a correlation with each other are reproduced by said respective loudspeakers and are collected by said at least one microphone, the method comprising: estimating individual transfer functions of said plurality of said audio transfer systems so as to set corresponding filter characteristics, respectively; producing echo cancel signals respectively by applying said set filter characteristics to corresponding ones of said multi-channel audio signals to be reproduced by said respective loudspeakers or; and subtracting said echo cancel signals from corresponding individual collected audio signals of said at least one microphone, thereby performing echo cancellation, wherein, reference signals are determined as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that among said multi-channel audio signals, for deriving estimated errors of said individual transfer functions of the respective audio transfer systems, thereby updating said corresponding filter characteristics to values that cancel the estimated errors, and wherein the correlation between said plurality of said low-correlation composite signals is detected and, when a value of said correlation is no less than a predetermined value, the updating of said filter characteristics is suspended.
 18. A transfer function calculation apparatus being associated to a space provided therein with a plurality of loudspeakers and at least one microphone for forming a plurality of audio transfer systems, and being capable of estimating individual transfer functions of said plurality of audio transfer systems, the apparatus comprising: an input section that inputs multi-channel audio signals from an outside, which have a correlation with each other, and which are reproduced by said respective loudspeakers and collected by said at least one microphone through the audio transfer systems; a providing section that provides reference signals as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that between said multi-channel audio signals; and a calculation section that estimates the individual transfer functions of the respective audio transfer systems based on the determined reference signals.
 19. A transfer function calculation apparatus as recited in claim 18, wherein the calculation section respectively derives the individual transfer functions of the respective audio transfer systems, using as the reference signals the set of the plurality of the low-correlation composite signals, such that calculation of respectively deriving the individual transfer functions of the respective audio transfer systems is based on a cross-spectrum calculation between the plurality of said low-correlation composite signals and individual collected audio signals of the at least one microphone.
 20. A transfer function calculation apparatus as recited in claim 19, wherein the providing section produces a plurality of uncorrelated composite signals mutually orthogonal as the reference signals by applying a principal component analysis to said multi-channel audio signals, such that the calculation section derives cross spectra based on said cross-spectrum calculation between said plurality of said uncorrelated composite signals and the individual collected audio signals of the at least one microphone, and ensemble-averages the respective cross spectra in a predetermined time period for deriving the individual transfer functions of said plurality of said audio transfer systems.
 21. A transfer function calculation apparatus being associated to a space provided therein with a plurality of loudspeakers and at least one microphone for forming a plurality of audio transfer systems through which multi-channel audio signals having a correlation with each other are reproduced by said respective loudspeakers and are collected by said microphones, and being capable of estimating individual transfer functions of said plurality of audio transfer systems, the apparatus comprising: a providing section that provides reference signals as a set of a plurality of low-correlation composite signals which correspond to signals obtained by suitably combining said multi-channel audio signals and which have a lower correlation with each other than that between said multi-channel audio signals; and a calculation section that estimates the individual transfer functions of the respective audio transfer systems based on the determined reference signals, wherein the calculation section respectively derives the individual transfer functions of the respective audio transfer systems, using as the reference signals the set of the plurality of the low-correlation composite signals, such that calculation of respectively deriving the individual transfer functions of the respective audio transfer systems is based on a cross-spectrum calculation between the plurality of said low-correlation composite signals and individual collected audio signals of the at least one microphone, and wherein the providing section combines said multi-channel audio signals through an arithmetic operation to produce said plurality of said low-correlation composite signals having a lower correlation with each other than that between said multi-channel audio signals, such that the calculation section derives cross spectra based on said cross-spectrum calculation between said plurality of said low-correlation composite signals and the individual collected audio signals of the at least one microphone, and ensemble-averages the respective cross spectra in a predetermined time period for deriving the individual transfer functions of said plurality of said audio transfer systems. 